GCSE Chemistry - Moles, Concentration & Volume Calculations

Summary

This video explains how to calculate the number of moles in an aqueous substance using the relationship between concentration, volume, and moles. It covers basic calculations, such as finding moles from concentration and volume, as well as more complex scenarios involving chemical reactions. The process involves converting units, calculating moles of known substances, using molar ratios, and determining the concentration of unknown substances. A practical example involving potassium hydroxide and sulfuric acid is used to illustrate these steps in detail.

Takeaways

• 😀 The equation links the number of moles of a substance to its concentration and volume. Volume is measured in decimeters cubed and concentration in moles per decimeter cubed.
• 😀 The formula can be written as n = c × v, where n is the number of moles, c is concentration, and v is volume.
• 😀 To find the moles in a solution, multiply the volume (in decimeters cubed) by the concentration (in moles per decimeter cubed).
• 😀 When given the volume in centimeters cubed, convert it to decimeters cubed by dividing by 1000.
• 😀 For example, 800 cm³ of sodium hydroxide with a concentration of 0.2 mol/dm³ results in 0.16 moles after conversion and calculation.
• 😀 To find the concentration of a solution, divide the number of moles by the volume in decimeters cubed.
• 😀 For a given problem, like determining the concentration of hydrochloric acid, you can solve it by dividing the moles by the volume.
• 😀 For more complex calculations, like with two reacting substances, start by determining the moles of the known substance.
• 😀 Use the mole ratio from the balanced chemical equation to determine the moles of the unknown substance.
• 😀 After finding the moles of the unknown substance, use the volume to calculate its concentration, ensuring all units are consistent (decimeters cubed).

Q & A

What units should be used for volume and concentration when using the equation to calculate moles?

Volume should be measured in decimeters cubed (dm³), and concentration should be in moles per decimeter cubed (mol/dm³).

What does the forward slash (/) in the concentration unit mean?

The forward slash (/) in the concentration unit means 'per,' indicating the ratio of moles to the volume in decimeters cubed.

How do you convert the volume from centimeters cubed to decimeters cubed?

To convert volume from centimeters cubed to decimeters cubed, divide the volume by 1000.

If we have 800 cm³ of sodium hydroxide with a concentration of 0.2 mol/dm³, how many moles of sodium hydroxide do we have?

First, convert the volume from 800 cm³ to 0.8 dm³. Then, multiply 0.8 by 0.2 mol/dm³, giving 0.16 moles of sodium hydroxide.

Given 1.5 dm³ of hydrochloric acid and 0.6 moles of hydrochloric acid, how do you calculate the concentration?

To calculate the concentration, divide the moles (0.6) by the volume (1.5 dm³). The concentration is 0.4 mol/dm³.

In a reaction between potassium hydroxide and sulfuric acid, how do you find the concentration of sulfuric acid?

First, calculate the moles of potassium hydroxide. Then, use the molar ratio to find the moles of sulfuric acid, and finally, divide the moles of sulfuric acid by its volume to find the concentration.

What is the first step in solving a reaction-related concentration problem?

The first step is to calculate the number of moles of the known substance using its concentration and volume.

In the potassium hydroxide and sulfuric acid example, how do you find the moles of potassium hydroxide?

To find the moles of potassium hydroxide, multiply the concentration (0.5 mol/dm³) by the volume (0.03 dm³), resulting in 0.015 moles.

What does the molar ratio tell you in the context of chemical reactions?

The molar ratio tells you how many moles of one substance react with a given number of moles of another substance. For example, a 2:1 ratio means two moles of one substance react with one mole of another.

How do you calculate the concentration of sulfuric acid in the given reaction example?

After calculating the moles of sulfuric acid (0.0075 moles), divide it by the volume of sulfuric acid (0.025 dm³). The concentration is 0.3 mol/dm³.