Avogadro’s number calculations for A-level Chemistry

Avogadros number calculations a level chemistry

Avogadro’s number calculations are pretty straight forward but many students are a little wary of them. I think the problem is a mixture of them not being covered at school, and them not coming up on the exams often enough to give you chance to get your head around them.

This tutorial will cover the equations you need to know, along with worked examples, and there’s a multiple choice question quiz to give you the opportunity to put your new knowledge into practice.

Avogadro’s number equations

Avogadro’s number is the number of particles in 1 mole of a substance: 6.02 x 1023 particles. You may be given this number to a different level of precision.

In this context, particles can be molecules, atoms or ions.

You need to know 2 equations that involve avogadro’s number:

  1. the number of particles = moles x avogadro’s number
  2. the mass of a particle = molar mass/avogadro’s number

You might need to use these equations as they are, or rearrange them. You may also need to use them in combination with other equations e.g. mass = moles x molar mass.

Calculating the number of particles

Here’s an example question:

How many atoms are in 0.500 moles of Argon. Avogadro’s number is 6.02 x 1023 particles. 

This is a nice straightforward question. The equation we need is the number of particles = moles x avogadro’s number

We know the number of particles (i.e. atoms) and avogadro’s number, so all we need to do is 0.5 x 6.02 x 1023 = 3.01 x 1023 

And that’s all you need to do.

Here’s a more difficult one

How many chloride ions are in 1.20 moles of MgCl2?

Again, we need to know the number of particles, so we’ll be using the number of particles = moles x avogadro’s number equation. But the problem is, the question asks about chloride ions, but the number of moles we have refers to the MgCl2, not the Cl ions. So before we can do anything else, we need to work out the number of moles of chloride ions. 

Here’s how we’ll do it: 1 mole of MgCl2 contains 2 moles of Cl ions, so as we have 1.20 moles of MgCl2 we have 2 x 1.20 = 2.50 moles of Cl.

We can now calculate the number of chloride ions: 2.40 x 6.02 x1023 = 1.44 x1024.


Here’s an example starting with mass

To work out the number of molecules (particles), we need to know the number of moles of SiCl4. We don’t have moles yet, but we do have mass so we can do moles = mass / molar mass

Moles = 18.2 / 170.1 = 0.107

Now we have the moles, we can work out the number of molecules:

Number of molecules = 0.107 x 6.02 x1023 = 6.44 x1022

Calculating the mass of a particle

Calculating the mass of a particle

if you need to calculate the mass of a particle, we need to use this equation: mass of a particle = molar mass / avogadro’s number.

When you use this equation, the units of the answer will be grams. If you need to give your answer in any other units you’ll need to do some converting.

Here’s an example:

What is the mass of a molecule of ethane?

Ethane has the formula C2H6, so the molar mass is (12×2) + (1×6) = 30.
Therefore the mass of a molecule of ethane = 30/6.02 x1023 = 4.98 x10-23

Avogadro’s number calculations quiz

It’s time to put your new knowledge of Avogadro’s number calculations into practice.

This quiz has 10 multiple choice questions. Some are a straightforward test of your knowledge, others are exam-style problem solving questions.

You’ll need a period table, so grab the AQA one here. Don’t worry if you’re using another exam board, this will be fine.

Avogadro’s number calculations quiz

1. How many atoms are in 0.600 moles of Helium?

Correct! Wrong!

2. How many atoms are in 0.200 moles of Cl2?

Correct! Wrong!

3. How many molecules are in 1.61 x10-3 moles of ethene?

Correct! Wrong!

4. How many iron ions are in 0.500 moles of Fe2O3?

Correct! Wrong!

5. How many molecules are present in 5.00 kg of O2?

Correct! Wrong!

6. A sample of SO2 contains 5.00 x1023 molecules. How many moles are present in this sample?

Correct! Wrong!

7. How many hydroxide ions are present in 10g of Barium hydroxide?

Correct! Wrong!

8. What is the mass of one atom of nitrogen?

Correct! Wrong!

9. What is the mass of 1 molecule of BF3?

Correct! Wrong!

10. 1 molecule of Cl2. has a mass of 1.16 10-22. Which isotopes are present in this molecule?

Correct! Wrong!

Avogadro's number calculations
Work through the examples above again
Good attempt
You've cracked it!