1.2 Water of crystallisation

Summary

This video by M sjcam explores the concept of water of crystallization, demonstrating the difference between hydrated and anhydrous salts using copper sulfate pentahydrate as an example. It outlines the experimental procedure to determine water content in a hydrated salt, including steps like measuring mass, heating to constant weight, and calculating moles to find the water of crystallization. The video concludes with an example calculation for barium chloride dihydrate and discusses the assumptions made during the experiment.

Takeaways

• 💧 The water of crystallization refers to the fixed number of water molecules in one formula unit of a salt.
• 🔬 Hydrated salts, such as copper sulfate pentahydrate, contain water molecules as part of their structure, whereas anhydrous salts do not.
• 🔥 Heating a hydrated salt can cause it to decompose into an anhydrous salt and water vapor.
• 📝 The naming of hydrated salts is based on the number of water molecules they contain; for example, 'pentahydrate' indicates five water molecules.
• 🔬 An experimental procedure to determine water of crystallization involves measuring the mass of a crucible, heating the sample, and weighing until a constant mass is achieved.
• ⚖️ The mass of the empty crucible and lid must be measured before adding the sample to determine the mass of the hydrated salt.
• 🔥 Heating the sample to constant mass is crucial to ensure all water of crystallization is driven off.
• 📉 The mass of water lost during heating represents the water of crystallization in the sample.
• 📊 Converting the mass of the anhydrous salt and the water lost into moles helps in determining the ratio of components in the hydrated salt.
• 🧪 The formula for the hydrated salt is derived from the mole ratio of the anhydrous salt to the water molecules.
• 🤔 Assumptions in the experiment include the loss of all mass being due to water, complete removal of water of crystallization, no water absorption by the crucible, and no further decomposition of the anhydrous salt.

Q & A

What is water of crystallization?

Water of crystallization refers to the fixed number of water molecules present in one formula unit of a salt.

What is the difference between copper sulfate pentahydrate and anhydrous copper sulfate?

Copper sulfate pentahydrate is the hydrated form of the salt with five water molecules attached, whereas anhydrous copper sulfate is the dehydrated form obtained after heating, which lacks the water molecules.

Why is the salt called copper sulfate pentahydrate?

The name 'copper sulfate pentahydrate' indicates that there are five water molecules associated with each formula unit of the salt, hence the term 'penta' for five.

What happens when copper sulfate pentahydrate is heated?

Upon heating, copper sulfate pentahydrate decomposes to form anhydrous copper sulfate and water vapor.

What is the purpose of heating to constant mass in the experimental procedure?

Heating to constant mass ensures that all the water of crystallization has been driven off and the mass of the anhydrous salt remains stable, indicating the end of the dehydration process.

How is the mass of an empty crucible and lid measured in the experimental procedure?

The mass of an empty crucible and lid is measured first as a baseline before adding the sample, to later determine the mass of the hydrated sample accurately.

What is the significance of recording the mass of the sample after each heating step?

Recording the mass after each heating step helps to monitor the loss of water and to confirm when the mass has stabilized, indicating that all water of crystallization has been removed.

How can the formula for a hydrated salt be determined from experimental data?

By comparing the masses of the hydrated sample, the water driven off, and the anhydrous salt, and then converting these masses to moles, the ratio of components in the hydrated salt can be determined, leading to the formula.

What is the formula for the hydrated salt BaCl2·xH2O derived from the given data?

The formula for the hydrated salt is determined to be BaCl2·2H2O, indicating barium chloride dihydrate.

What assumptions are made in the experiment to determine water of crystallization?

The assumptions include that all mass lost is due to water loss, all water of crystallization is driven off, the crucible does not absorb water, and the anhydrous barium chloride does not decompose further.

Outlines

00:00

💧 Water of Crystallization in Salts

This paragraph introduces the concept of water of crystallization, which is the fixed number of water molecules associated with a salt. It explains the difference between hydrated and anhydrous forms of salts, using copper sulfate pentahydrate as an example. The hydrated salt retains its water molecules, while the anhydrous form does not. The process of heating copper sulfate pentahydrate to form anhydrous salt and water vapor is described, along with a caution about overheating that could lead to further decomposition.

Keywords

💡Water of Crystallization

Water of crystallization refers to the fixed number of water molecules that are part of the crystal structure of a hydrated salt or compound. In the video, this concept is central to understanding the difference between hydrated and anhydrous salts, such as copper sulfate pentahydrate and copper sulfate. The water molecules are integral to the compound's chemical identity and properties.

💡Hydrated Salt

A hydrated salt is a compound that includes water molecules as part of its crystal structure. In the script, copper to sulfate pentahydrate is an example of a hydrated salt, where 'penta' indicates the presence of five water molecules per formula unit of the salt. This term is essential for distinguishing the salt before and after the dehydration process.

💡Anhydrous Salt

An anhydrous salt is a salt that does not contain any water of crystallization. The video describes the transformation of copper sulfate pentahydrate into anhydrous copper sulfate upon heating, which results in the loss of water molecules and a change in color from blue to white.

💡Decomposition

Decomposition in the context of chemistry refers to the process where a compound breaks down into two or more simpler substances. In the video, the decomposition of copper sulfate pentahydrate is highlighted, resulting in the formation of anhydrous copper sulfate and water vapor.

💡Crucible

A crucible is a container made from a material that can withstand very high temperatures, often used for melting or decomposing substances. In the script, a crucible is used to heat the hydrated salt to drive off the water of crystallization, demonstrating the experimental procedure for determining the water content in a hydrated salt.

💡Constant Mass

Achieving a constant mass in an experiment indicates that the process of heating or decomposition has completed, and no further change in mass is occurring. The video script describes the process of heating the crucible until the mass of the sample remains constant, signifying the end of the dehydration process.

💡Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is used to convert the mass of a substance into the amount in moles, as shown in the script when determining the moles of anhydrous barium chloride and water.

💡Moles

Moles are a unit of measurement used in chemistry to express amounts of a chemical substance. In the script, the mass of the anhydrous sample and the mass of water are converted into moles to determine the ratio of barium chloride to water in the hydrated salt.

💡Barium Chloride Dihydrate

Barium chloride dihydrate is a specific example of a hydrated salt mentioned in the script, where 'di' indicates the presence of two water molecules per formula unit of barium chloride. The determination of this formula is part of the experimental procedure described in the video.

💡Assumptions

Assumptions in a scientific experiment are the presuppositions made to simplify the process or to make the experiment feasible. The script outlines several assumptions made during the dehydration experiment, such as the loss of all mass being due to water, and the crucible not absorbing water.

💡Deacon's Process

Deacon's process is a method for producing chlorine gas from hydrochloric acid using an activated carbon catalyst. In the script, it is mentioned as a potential further decomposition process for anhydrous barium chloride, although it is not the focus of the video.

Highlights

Introduction to the concept of water of crystallization in salts.

Explanation of the difference between hydrated and anhydrous salts, illustrated with copper sulfate pentahydrate and copper sulfate.

Demonstration of the decomposition of copper sulfate pentahydrate upon heating to form anhydrous salt and water vapor.

Description of the experimental procedure to determine the water of crystallization, starting with measuring the mass of an empty crucible and lid.

Step-by-step guide on heating the crucible to drive off water, ensuring gas can escape, and cooling to re-weigh until mass remains constant.

Calculation of the water of crystallization for barium chloride dihydrate using mass measurements from an experimental setup.

Method to determine the mass of the hydrated sample by subtracting the mass of the crucible and lid from the total mass.

Process for calculating the mass of water driven off by heating the sample to constant mass.

Conversion of mass to moles to find the ratio of anhydrous barium chloride to water.

Derivation of the chemical formula for the hydrated salt, BA CL2 X H2O, based on the mole ratio.

Naming of the salt as barium chloride dihydrate based on the determined formula.

Assumptions made during the experiment, including the loss of all mass as water and complete removal of crystallization water.

Assumption that the crucible does not absorb water during the heating process.

Assumption that the anhydrous barium chloride does not decompose further.

Importance of careful heating to prevent further decomposition of the anhydrous salt.

Practical applications of understanding water of crystallization in chemical analysis and experiments.

Theoretical implications of the experiment for the study of salt hydration and dehydration processes.

Transcripts

this is M sjcam in this video I'll be

looking at the water of crystallization

the water of crystallization is the

fixed number of water molecules present

in one formula unit of a salt in this

picture on the left we have the hydrated

salt copper to sulfate pentahydrate on

the right we have the unhide result

copper to sulfate

the difference between the two salts is

the salt on the Left still has the five

water molecules attached to the salt

these five water molecules are the water

of crystallization the hydrated salt is

named according to how many water

molecules there are in the salt so in

this case the salt is named copper to

sulfate pentahydrate because there are

five water molecules upon heating the

blue copper sulfate pentahydrate

decomposes forming the white anhydrous

salt and water vapor on the Left we can

see the blue hydrated salt being heated

forming the white and hydro salt and

here we have the equation for the

decomposition of copper sulfate

pentahydrate into the anhydrous salt and

water vapor care must be taken as

overheating of the unhide result can

cause further decomposition to take

place next we look at the experimental

procedure for determining the water of

crystallization the first step is to

measure the mass of an empty crucible

and lid step two is at a known mass of

the sample and record the mass step

three is heat the crucible for five

minutes holding the lid at an angle so

the gas can escape step four is after

cooling re-weigh the crucible lid and

contents and Step five is repeat steps 3

and 4 until the mass remains constant

this is known as heating to constant

mass so next we'll determine the water

of crystallization for the hydrated salt

ba CL 2 X h2o

from the following data so in this table

we have the mass of crucible and lid the

mass of crucible lid and sample the mass

of crucible lid and

after first heating and the mass of the

crucible lid and sample after second

heating note that the mass of the

crucible lid and sample after first

heating and second heating are the same

this is what is meant by heating to

constant mass so first we'll determine

the mass of the hydrated sample to do

this we subtract the mass of the

crucible and lid from the mass of the

crucible lid and sample which gives us

five point zero zero grams next we

determine the mass of water driven off

when the sample was heated to do this we

subtract the mass of crucible lid and

sample after heating from the mass of

the crucible lid and sample which gives

us zero point seven four grams and

finally we can determine the mass of the

anhydrous ba CL - to do this we subtract

the mass of water driven off when the

sample was heated from the mass of the

hydrated sample which gives us four

point seven six grams next we need to

convert from mass in grams to amount in

moles so we divide the mass of the un--

hydra's sample by its molar mass which

gives us zero point zero two moles next

for the water we divide the mass of

water driven off by the molar mass of

water which gives us zero point zero

four moles we then divide each of these

values by the smallest to give us the

lowest whole number ratio so that's one

for the anhydrous bacl two and two for

the h2o this gives us the formula for

the hydrated salt which is BA CL - 2 h2o

the name of the salt is barium chloride

dihydrate finally we look at the

assumptions made in the experiment the

first assumption is that all mass lost

is due to the loss of water the second

is that all water of crystallization is

driven off the third is that the

crucible does not absorb water and the

last assumption is that the anhydrous

barium chloride does not Deacon

pose further