Acid-Base Titrations & Standard Solutions | A-level Chemistry | OCR, AQA, Edexcel

Summary

This video series educates viewers on titrations, a volumetric analysis technique to determine chemical unknowns like concentration and molar mass. It covers the titration process, using standard solutions, and calculations involved. The script guides through preparing standard solutions, conducting titrations, and analyzing results. It also includes practical examples, such as making a sodium hydroxide solution and calculating unknown concentrations or molar masses, enhancing understanding of titration applications.

Takeaways

• 🔬 **Titrations Defined**: Titrations are a form of volumetric analysis used to determine unknown chemical properties by reacting a known volume and concentration of one solution with a measured volume of another.
• 📊 **Standard Solutions**: Standard solutions, with known concentrations, are crucial in titrations for calculating unknowns such as concentration, molar mass, and chemical formulas.
• 🧪 **Titration Process**: The process involves adding a measured volume of substance X to a flask, adding an indicator, and then adding substance Y from a burette until the endpoint is reached, allowing for the calculation of unknowns.
• 💧 **Indicator Role**: Indicators, chosen based on the chemicals involved, are added to the flask to signal the endpoint of a titration.
• 🔋 **Burette Usage**: A burette is used to add the solution Y into the flask containing X, with careful control over the addition until the endpoint is reached.
• 🔎 **Calculating Unknowns**: By analyzing titration results, one can calculate various unknown chemical properties, guided by the principles of stoichiometry.
• ⚖️ **Making Standard Solutions**: The creation of a standard solution involves dissolving a weighed amount of solute in a solvent, transferring to a volumetric flask, and making up to the mark with solvent.
• 📐 **Concentration Concept**: Concentration measures how much solute is dissolved in a given volume of solvent, which is fundamental to understanding titration calculations.
• 🔄 **Solution Mixing**: After preparing a standard solution, it is thoroughly mixed by inverting the flask to ensure homogeneity.
• 📚 **Titration Calculations**: Calculations in titration involve determining the concentration of unknown solutions, molar masses, and other properties using the reaction stoichiometry and volumes measured.
• 📉 **Error Identification**: The script highlights common errors in preparing standard solutions and suggests improvements, such as using a weighing boat and a volumetric flask for accuracy.

Q & A

What is a titration?

A titration is a form of volumetric analysis where a known volume and concentration of one solution reacts with a measured volume of another solution to determine chemical unknowns such as concentration, molar mass, formula, and water crystallization.

What is a standard solution?

A standard solution is a solution with a known concentration used in titrations to calculate chemical unknowns.

How do you make a standard solution?

To make a standard solution, you first weigh the required mass of the solute, dissolve it in a solvent in a beaker, then transfer the solution into a volumetric flask. Rinse the beaker and add the washings to the flask. Add solvent to the flask without exceeding the graduation line, then adjust the volume by adding solvent drop by drop until the meniscus is at the graduation line. Finally, mix the solution thoroughly by inverting the flask.

What is the purpose of an indicator in titration?

An indicator in titration is used to signal the endpoint of the reaction, indicating when the titration is complete.

How do you calculate the number of moles required for a standard solution?

The number of moles required is calculated by multiplying the desired concentration by the volume in decimeters cubed.

What is the molar mass of sodium hydroxide?

The molar mass of sodium hydroxide (NaOH) is approximately 40 grams per mole, calculated by adding the atomic masses of sodium (23), oxygen (16), and hydrogen (1).

How do you calculate the mass of a solute needed for a standard solution?

The mass of the solute needed is calculated by multiplying the number of moles by the molar mass of the substance.

What is the reaction between hydrochloric acid and sodium hydroxide?

The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) produces sodium chloride (NaCl) and water (H2O).

How do you find the concentration of an acid using titration?

To find the concentration of an acid using titration, you calculate the moles of the base that reacted, use the reaction stoichiometry to find the moles of acid, and then divide the moles of acid by the volume of acid used to get the concentration.

What is the significance of the meniscus in making a standard solution?

The meniscus is significant in making a standard solution because it indicates the correct volume of solution in the volumetric flask. The bottom of the meniscus should align with the graduation line to ensure the accurate volume.

Why is it important to mix the solution thoroughly after making a standard solution?

It is important to mix the solution thoroughly after making a standard solution to ensure homogeneity and that the concentration of the solute is uniform throughout the solution.

Outlines

00:00

🧪 Introduction to Titrations and Standard Solutions

The paragraph introduces a series of videos focusing on titrations, a form of volumetric analysis used to determine chemical unknowns such as concentration, molar mass, formula, and water crystallization. It explains the titration process, which involves reacting a known volume and concentration of one solution with a measured volume of another. The paragraph also discusses standard solutions, which have a known concentration and are crucial for titrations. The process of making a standard solution is outlined, including weighing the solute, dissolving it in a solvent, transferring it to a volumetric flask, adding solvent up to a graduation line, and mixing thoroughly. The calculations involved in preparing a standard solution are also covered, such as determining the number of moles required and calculating the mass of the solute needed.

05:02

🔍 Calculating Concentration and Molar Mass in Titrations

This paragraph delves into titration calculations, explaining how to find unknowns like concentration and molar mass. It provides a step-by-step guide to calculating the concentration of hydrochloric acid when titrating sodium hydroxide, emphasizing the importance of understanding the reaction equation and the stoichiometry involved. The paragraph also demonstrates how to calculate the molar mass of an acid using titration data, showing the process of determining moles of acid in a solution and then using that to find the molar mass based on the mass of the acid used.

10:02

📚 Improving Standard Solution Preparation

The focus of this paragraph is on improving the method of preparing standard solutions. It points out errors in a student's approach, such as measuring sodium hydroxide directly on a balance, and suggests using a weighing boat to ensure no residue is left behind. It also recommends using a volumetric flask for accurate solvent volume measurement and inverting the flask to mix the solution, rather than stirring with a glass rod, to prevent residue on the rod.

15:02

📈 Determining Chemical Quantities in Titration

The paragraph discusses how to calculate the amount of sodium hydroxide and sulfuric acid used in a titration reaction. It explains the stoichiometry of the reaction and how to use the concentration and volume of the solutions to calculate moles. The summary also covers calculating the concentration of sulfuric acid used in the titration, emphasizing the importance of understanding the reaction's stoichiometry and applying it to find the unknown concentration.

20:03

🧐 Writing Reaction Equations and Calculating Concentrations

This paragraph instructs on writing balanced chemical equations for titration reactions, using the example of barium hydroxide reacting with nitric acid. It covers how to balance the equation and calculate the initial concentration of the acid based on the volume and moles of barium hydroxide used. The paragraph also highlights the importance of using appropriate apparatus like a burette and pipette for accurate volume measurements and the necessity of adding an indicator to determine the endpoint of a titration.

25:03

🎓 Engaging with A-Level Chemistry Tutorials

The final paragraph serves as a call to action for viewers to join the tutorial series for A-Level chemistry. It emphasizes the engaging and bite-sized nature of the video tutorials, aiming to make understanding A-Level chemistry more accessible and enjoyable.

Keywords

💡Titration

Titration is a laboratory method used to find the concentration of an unknown solution. It involves gradually adding a solution of known concentration to the unknown solution until the reaction is complete, as indicated by a color change from an added indicator. In the script, titration is the main focus, with detailed steps on how to perform it and its applications in determining chemical unknowns such as concentration and molar mass.

💡Standard Solution

A standard solution is a solution with a precisely known concentration, which is crucial for titration as it allows for accurate measurement of unknown substances. The script describes the importance of standard solutions and outlines the process for creating one, emphasizing the meticulous steps required for accuracy.

💡Concentration

Concentration in the context of the video refers to the amount of solute dissolved in a given volume of solvent, typically expressed in moles per liter. It is a fundamental concept in chemistry and is essential for titration, as it helps determine the quantities of substances reacting in a chemical equation. The script explains how to calculate the concentration of solutions involved in titrations.

💡Molar Mass

Molar mass is the mass of one mole of a substance and is calculated by adding the atomic masses of all the atoms in a molecule. The script uses molar mass in calculations to find the mass of a substance needed for preparing a standard solution or to determine the molar mass of an unknown acid through titration.

💡Volumetric Flask

A volumetric flask is a piece of laboratory glassware used to prepare solutions of a precise, known volume. The script mentions the use of a volumetric flask in the process of making a standard solution, highlighting its importance in ensuring the solution has the exact volume needed for accurate titrations.

💡Moles

Moles are a measure in chemistry used to express the amount of a substance, based on the number of atoms or molecules. The script explains how to calculate the number of moles required for a solution, which is a key step in preparing standard solutions and performing titrations.

💡Indicator

An indicator is a substance added to a solution to signal the endpoint of a titration by changing color. The script mentions that an appropriate indicator is added to the flask before starting a titration, emphasizing its role in accurately determining when the reaction is complete.

💡Burette

A burette is a piece of laboratory equipment used to dispense liquids, particularly in titrations. The script describes filling a burette with solution Y and using it to add precise amounts of the solution to substance X in the flask, illustrating its critical role in the titration process.

💡End Point

The end point in titration is the point at which the reaction between the substances is complete. The script explains that the process of adding solution Y continues until the end point is reached, as indicated by the color change from the indicator, marking the completion of the titration.

💡Conical Flask

A conical flask is a type of glassware with a narrow neck and a conical shape, used in titrations to contain the solution being tested. The script mentions adding the first substance X to a conical flask, where it is then reacted with solution Y from the burette.

💡Molarity

Molarity is a measure of concentration, defined as the number of moles of solute per liter of solution. The script uses molarity in calculations to determine the concentration of acids or bases used in titrations, which is essential for calculating unknowns like the concentration of the titrant or titrate.

Highlights

Introduction to the concept of titrations and their role in volumetric analysis.

Explanation of how titrations can be used to determine chemical unknowns such as concentration, molar mass, formula, and water crystallization.

Definition and importance of standard solutions in titration processes.

Description of the titration process involving a pipette, conical flask, and buret.

The role of indicators in titration and their dependence on the chemicals used.

Step-by-step guide on how to make a standard solution.

Explanation of concentration in terms of solute and solvent.

Instructions on weighing out the solute for making a standard solution.

Details on transferring the solution to a volumetric flask and the importance of not wasting any solution.

The correct method for filling a volumetric flask to the graduation line.

The necessity of thoroughly mixing the solution in a volumetric flask.

Initial calculations required before making a standard solution, including the number of moles and molar mass.

Worked example of calculating the mass of sodium hydroxide needed for a standard solution.

Titrations analysis and the calculations involved to find unknowns such as concentration and molar mass.

Calculation of an unknown concentration using the titration data of sodium hydroxide and hydrochloric acid.

Determining the molar mass of an acid using titration with sodium hydroxide.

Common errors in making a standard solution and suggestions for improvement.

Correct method for making a standard solution of magnesium hydroxide.

Titration between sulfuric acid and sodium hydroxide with calculations for moles of reactants.

Writing a balanced chemical equation for a titration reaction between barium hydroxide and nitric acid.

Calculating the initial concentration of an acid using titration with barium hydroxide.

Identification of apparatus used for accurately measuring volumes in titration, such as a burette and pipette.

Explanation of the necessity of adding an indicator for successful titration.

Transcripts

hi guys in the next series of videos

we're going to take a look at what are

titrations looking at the titration

process an introduction to standard

solutions looking at how we make

standard solutions titration

calculations an exam style question and

finally a summary

so let's have a look at what titrations

are

well titrations are a form of volumetric

analysis

where a known volume and concentration

of a solution is reacted with a measured

volume of a solution the results the

titration can be analyzed and can be

used to find chemical unknowns

these unknowns can include concentration

molar mass

formula

and water crystallization

so before we have a look at the

titration process let's have a look at

what we use

we use something called a standard

solution this is a solution with a known

concentration and is used in our

titrations

we'll take a closer look at the method

used in making a standard solution in a

later part to this video

let's first of all take a brief look at

the titration process

the first step is to use a pipette to

add a measured volume of your first

substance x to a conical flask

we then add a suitable indicator to the

flask

this indicator depends on the chemicals

you're using

we then fill the buret

with the other solution y

we then open the valve on our buret to

allow a small quantity of white to flow

into our flask of x

you can see here that we're turning that

valve to allow y to flow out and into

our conical flask of x

we repeat this process until the end

point is reached

and we can then measure the volume of y

that has been added

we can then calculate the unknowns by

analyzing the results of our titration

so what are standard solutions well

standard solutions as we mentioned in

the previous part of this video are

solutions of a known concentration

they're used in titrations which allow

us to calculate chemical unknowns

so first of all let's quickly recap the

concept of concentration and battle

solvents and solutes

the solute is dissolved within our

solvent and the concentration is a

measure of how much solute is dissolved

in a given volume of the solvent

so now we've recapped the idea of

concentration let's take a look at the

steps involved in making a standard

solution

the first step is to carefully weigh out

the required mass of our solute

the second step is to dissolve the

solute in our chosen solvent in a beaker

the third step is to transfer the

solution that we've made into a

volumetric flask

we then rinse the beaker with our

initial solvent that's the solvent that

we've chosen

and we add the washings into the

volumetric flask making sure nothing is

washed away or poured down a sink and

wasted

in the fourth step we add some of our

solvent to the volumetric flask making

sure not to fill up to the graduation

line that's a line that you'll see on

the neck of a volumetric flask

the fifth step is to add the solvent

drop by drop until the bottom of the

meniscus is sitting on that graduation

line ensuring we have just the right

volume of solution

it's important to not allow the solution

to fill above the line if you do you'll

have to start the process all over again

the final step is step six to mix the

solution thoroughly and this is done by

inverting your flask multiple times

making sure you put the stopper in

tightly first of all

so we've had a look at the method of

making a standard solution let's have a

look at the calculations we do before we

start making our standard solution

the first step is to work out the number

of moles required that's the number of

moles of our solute required

we then calculate the molar mass of the

required substance and we can calculate

the mass of the substance that is

required that's the mass we're going to

weigh out

so let's have a look at a worked example

so you can really understand these

initial calculations

we want to make a 250 centimeter cubed

solution of sodium hydroxide with a

concentration of 0.1 mole per decimeter

cubed

so the first step is to work out the

number of moles required

so

using our equation pyramid the number of

moles is equal to the concentration

multiplied by the volume we want to know

the number of moles so that's the

concentration multiplied by the volume

is the arrangement we're going to use

the information we're given was that the

concentration required as 0.1 moles per

decimeter cubed and the volume was

250 centimeters cubed that's equal to

0.25 decimeters cubed

so that'll be the number of moles is

equal to 0.1 multiplied by 0.25

giving us

0.025 moles required

now to calculate the molar mass

we're producing a solution of sodium

hydroxide so the molar mass of sodium

hydroxide is what we're going to have to

calculate

if we look at our periodic table we can

see sodium has a molar mass of 22.99

which we can round to 23.

hydrogen 1 and oxygen 15.99 which are

round to 16.

so that will be 23 plus 16 plus 1 a

molar mass of 40.

in our final step we're calculating the

mass required we know the number of

moles is equal to

0.025 moles and we know the molar mass

is equal to 40.

so we can look at our equation that the

number of moles is equal to the mass

divided by the molar mass the equation

rearrangement we're going to use is that

the mass is equal to the number of moles

multiplied by the molar mass so that's

0.025 multiplied by 40

giving us a mass of one gram that's

required

so let's have a look at titration

analysis and the calculations that we

can carry out

in general these calculations can follow

certain steps and i'll show you how our

calculations can be broken down in these

easy to follow steps

calculations can be used to find

unknowns

these unknowns could be

the concentration of the solution the

molar mass and many other things

exam questions will usually guide you

through it'll give you small steps to

follow so don't be worried if a big

titration calculation comes up

let's have a look at some of the

calculations that we can do

the first calculation we're going to

look at is the calculation of an unknown

concentration

25 centimeters cubed of a 0.1 mole per

decimeter cubed solution of a base

sodium hydroxide is titrated with 22.5

centimeters cubed of an acid

hydrochloric acid

what is the concentration of the acid

involved in this titration

so we're looking to find out the

concentration of the acid that's our

unknown so let's answer this question

looking at the steps involved

the first step is the reaction equation

we know that our acid hydrochloric acid

is reacting with our base sodium

hydroxide and we know that acids and

bases react together to form a salt in

this case sodium chloride and water now

we can check to see our equation is

balanced and it indeed is

the second step is to calculate the

amount in moles of base that has reacted

so what do we know we know the volume of

base was 25.0 centimeters cubed that's

equal to

0.025 decimeters cubed

we know the concentration of our base

was 0.1 moles

per decimeter cubed

now using our equation pyramid the

number of moles is equal to the

concentration multiplied by the volume

the number of moles is equal to

0.1 multiplied by naught 0.025

to give us 0.0025

moles of base

now to calculate the amount in mole of

acid that was used

looking at the reaction equation up here

we can see that one mole of hydrochloric

acid reacts with one mole of sodium

hydroxide it's a one to one ratio

so one to one means that

0.00025 moles of our base will react

with

0.0025 moles of our acid

meaning that the number of moles was

0.0025

moles

so now to calculate the concentration

again write down what we know we know

the number of moles is equal to 0.025

we know the volume of our acid used was

22.5 centimeters cubed that was given to

us in the question that is equal to

0.0225

decimeters cubed

now if we look at our equation pyramid

the number of moles is equal to the

concentration times by the volume

the rearrangement of this equation we're

going to use is that the concentration

is equal to the number of moles divided

by the volume so that is 0.0025

divided by

0.0225

to give us a concentration of 0.111

moles per decimeter cubed

so now we've had a look at finding an

unknown concentration let's have a look

at a slightly different calculation

calculation to find an unknown molar

mass

1.5 grams of an acid was dissolved in

water in order to make a solution with a

volume of 250 centimeters cubed

25 centimeters cubed of the solution of

this acid hx was titrated with 22.5

centimeters cubed of a base sodium

hydroxide which had the concentration of

0.1 moles per decimeter cubed

what is the molar mass of the acid

involved in this titration

so the first step again is to write out

our reaction equation we have our acid h

x now don't worry that this acid hx

containing x is a general element

isn't specific it's just used to

represent an acid here

it's reacting with a base sodium

hydroxide we know acid plus base equals

salt plus water the salt is going to be

n a

x and we're going to form a water

checking to see our reaction is balanced

which is indeed is

we can then move on to step two which is

quite similar to the step two of our

previous calculation

calculate the amount in moles of base

that has reacted

so let's write down what we know

we know the concentration is equal to

0.1 moles per decimeter cubed as we're

given that in the question

and we know the volume of our base that

was used was

22.5 centimeters cubed that is equal to

naught point naught two to five

decimeters

cubed so we can write out our reaction

pyramid that the number of moles equals

the concentration multiplied by the

volume to see that the rearrangement we

need to use is the number of moles is

equal to the concentration multiplied by

the volume

the concentration is naught point one

moles per decimeter cubed and the volume

is 0.025

giving us no point not not two to five

moles of base that has been used

so now for the third step to calculate

the amount and mole of acid that was

used to make the initial solution

so if we have a look at our reaction

equation we can see that one mole of hx

reacts with one mole of sodium hydroxide

so that's a one-to-one ratio

that means that

0.025 moles of our sodium hydroxide will

react with

0.00225 moles of our acid

hx now importantly this is telling us

that there is

0.0025 moles of hx in the 25 centimeter

cubed sample we want to know how much

there was in the initial 250 centimeter

cubed solution now we know that 250

divided by 25 is equal to 10. so looking

at the volume that's 10 times as much

meaning there's going to be 10 times as

many moles so that's

0.00025 times 10

giving us

0.025

moles of acid in the 250 centimeter

cubes initial solution

now the final step to calculate the

molar mass of the acid that we used

well we know the number of moles is

0.025 we've just calculated that and we

know the mass of our acid was 1.5 grams

we were given that in the question so

using our reaction pyramid that the

number of moles is equal to the mass

over the molar mass we can work out that

the molar mass will be the mass divided

by the number of moles that's 1.5

divided by 0.02

to give us a molar mass of 66.667

grams per mole

a student decides to make up a standard

solution of sodium hydroxide she

carefully measures out 10 grams directly

onto a balance she adds this to a

conical flask with a hundred mils of

water and stirs using a glass rod

part a the student made a number of

errors in her method how could she

improve her method

so for three marks we should probably

give about three errors if we can find

them

so let's take a look at her method

it says here that

student measures out 10 grams of sodium

hydroxide directly onto a balance

it's probably better for the student to

use a weighing boat to ensure that no

residue is left behind as a weighing

boat can easily be washed out to ensure

that all of our chemical reactant is

being used

so the first improvement is she could

use a weighing boat

in the method we're then told that she

adds this to a conical flask

the student could instead use a

volumetric flask to ensure that an

accurate solvent volume is measured

and for our third and final suggestion

of improvement the method says the

student stirred using a glass rod

alternatively the student could invert

her flask in this case her volumetric

floss which we said she should use

to ensure that no residue is left on the

glass rod

so for each of those corrections we

receive one mark she could use a

weighing brake she can use a volumetric

flask and she can invert the flask

rather than staring with her glass rod

moving on to part b

having learnt the correct way of making

up a standard solution the student

decides to make up a standard solution

of mgoh2 magnesium hydroxide the student

wants to obtain one liter of 0.1 moles

per decimeter cubed solution

what mass of magnesium hydroxide should

she use

let's first of all draw out the pyramid

that we're going to need we know that

the number of moles is equal to the

concentration multiplied by the volume

so we can go ahead and calculate the

number of moles of magnesium hydroxide

in the solution

so we know the concentration is 0.1

moles per decimeter cubed and we know

the volume is one liter that's one

decimeter

so that gives us naught point one moles

so now we can go ahead and use our

second pyramid

number of moles is equal to the mass

divided by the molar mass

we know the number of moles is equal to

0.1

we want to know the mass and we can

calculate the molar mass if we take a

look at our periodic table we can see

that we can find all the components we

need we want to find out the molar mass

of magnesium hydroxide

mgoh2 we can see that we have hydrogen

over here with a mass of 1

oxygen over here with a mass of 16 and

magnesium over here the mass of 24.3

so the molar mass will be

24.3

plus

sixteen times two as we have two oxygens

plus one times two for our two hydrogen

atoms to give us a total

of fifty eight point three that is our

molar mass of magnesium hydroxide

so putting that into our equation

we can see

that the mass is equal to the number of

moles 0.1 multiplied by 58 58.3 our

molar mass to give us 5.83

grams that we require

so the answer is 5.83 grams of magnesium

hydroxide

so this question holds two marks the

first comes from correctly calculating

the number of moles of magnesium

hydroxide

in our solution and the second from a

correct final answer of the mass of

magnesium hydroxide we

need so let's go ahead and have a look

at question two a titration between

sulfuric acid and sodium hydroxide is

carried out the equation for the

reaction is shown below we're given the

equation here

25 centimeters cubed of 0.04

naught multiple decimeter cubed of

sodium hydroxide is neutralized by 17.5

centimeters cubed dilute sulfuric acid

part a asks us to calculate the amount

in moles of sodium hydroxide used in

titration

this is very similar to the second step

of the equations that we were doing

before

so write down what we know

we know that the concentration is equal

to 0.0440

moles per decimeter cubed and where the

volume is

25.0 centimeters cubed which is

5 0.025

cubed

we can look at our triangle pyramid

which is the number of moles equal to

the concentration multiplied by the

volume so n is equal to c times v

which is

0.044

multiplied by 0.025

to give us 0.0011

[Music]

moles of sodium hydroxide used

so now let's look at part b

calculate the amount in moles of h2so4

sulfuric acid used in the titration

so in order to do this we need to look

at our reaction equation

now we can see that two moles of sodium

hydroxide will react with one mole of

sulfuric acid so that's a two to one

ratio

so

if we have 0.0011

moles of our sodium hydroxide

it's going to react with half as much

sulfuric acid so that's

0.0011 divided by 2 to give us 0.00055

moles

of sulfuric acid

part c our final part is to calculate

the concentration in moles per decimeter

cubed of h2so4 sulfuric acid used in

titration

again as we always do write down what we

know we know the number of moles is

equal to 0.00055

moles as we've just worked out and in

the question we're told the volume is

17.5

centimeters cubed that's equal to

0.0175 decimeters cubed

so our equation pyramid shows us the

number of moles equals the concentration

multiplied by the volume we want to know

the concentrations the rearrangement

that we'll use is that the concentration

is equal to the number of moles divided

by the volume so that is 0.00055

divided by 0.01

to give us a concentration of 0.03143

moles

per decimeter cubed

question three a student carries out the

titration using barium hydroxide and

nitric acid

in part a we're asked to write an

equation for the reaction taking place

so we know our two reactants are barium

hydroxide

and

nitric acid

now we're going to form

salt and water our salt being barium

nitrate

so now we've written our equation we

need to go ahead and check to balance it

so on both sides of the equation we have

one barium

on the left hand side we have one

nitrogen on the right hand side in our

product we have two so we can balance

that by putting a two in front of our

nitric acid to check our hydrogen atoms

we have two here and two here that's

four on the reactant side the left hand

side and we have

only two on this side so we'll put a two

in front of our water molecule so now we

have four on both sides to check our

oxygen atoms there's two here and six

here that's eight and we have six here

and two here eight again so that is our

correct and balanced equation and for

that we get one mark

in part b we're told that 50 ml of

barium hydroxide a concentration of 0.5

moles per decimeter cubed are required

to fully titrate 100 ml solution of our

acid

we're asked what is the initial

concentration of the acid

so the first thing we're going to do is

write out the pyramid we're going to

need

we know that the number of moles is

equal to the concentration times the

volume of our solution

so we're going to work out the number of

moles of barium hydroxide that have been

used in the titration so the number of

moles is equal to the concentration

multiplied by the volume the

concentration is 0.5 moles per decimeter

cubed we have 50 mils so we're going to

multiply that by

50 over 1000 converting the mils there

to decimeters cubed

giving us not

.025 moles

now if we take a look at the equation

that we previously wrote we can see that

one mole of barium hydroxide reacts with

two of nitric acid

so that's one to two therefore if we

have

0.025 moles of our barium hydroxide

that's going to react with

0.05 moles of our nitric acid so we know

we have 0.05 moles of nitric acid that's

reacting initially

we know there's a 100 mils of the

solution so let's go ahead and quickly

note down what we know we know the

number of moles is 0.05

we know the volume is 100 ml so that's

100 over 1000 converting to decimeters

cubed giving us 0.1 decimeters cubed

now using the same pyramid we can

calculate the concentration see that the

concentration is the number of moles

divided by the volume so that's

0.05 divided by 0.1 to give us

0.5

moles per decimeter cubed

so this question holds three marks the

first mark is given for correctly

calculating the number of moles of

barium hydroxide

the second for applying that and using

that to calculate the number of moles of

acid

the third and final mark for correctly

calculating the initial concentration of

the acid remembering to give units part

c the last part of our question asked us

to name two pieces of apparatus that are

used to accurately measure the volume of

solutions in a titration and we're asked

what other chemical substance must be

added to the reaction flask for

titration to be successful so if we deal

with the first part of the question to

name two pieces of apparatus that are

used to accurately measure the volume of

solution in a titration this could be a

burette and also a pipette which are two

important pieces of apparatus that you

will have used that are used to

accurately measure volumes of solution

often small volumes

now to deal with the second part of the

question

what chemical substance must be added

well we know that it's important to use

an indicator because this will tell us

when we have reached the endpoint of our

titration

or when we are near it

there are a variety of different

chemical indicators that have been used

we're not asked to give a suggestion of

one here just to explain what chemical

supplements would be added but

some questions may ask you to go into

more detail and suggest an indicator

such as methyl orange or phenolphthalein

this question holds three marks

you get one mark for each suggestion of

a correct piece of apparatus and the

third and final mark comes from stating

it's an indicator that needs to be added

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