What is a mole?

In the field of chemistry, a ‘mole’ is a unit of measurement for the amount of substance. 1 mole contains approximately 6.022 x 1023 atoms, molecules, ions, or other particles. As chemical reactions can take place with solid, liquid and gaseous substances, it is essential that IGCSE chemistry students are able to convert between moles and other units (e.g. grams, cm³ and dm³). To do so is crucial when answering IGCSE chemistry questions that involve mole calculations.

The first step for those studying IGCSE chemistry is to memorise these formulae:

$Concentration \,(mol/dm^{3})=\dfrac{Number \,of \,moles \,(mol)}{Volume \,of \,solution \,(dm^{3})}$

$Number \,of \,moles \,(mol)=\dfrac{Mass \,of \,the \,solid \,(g)}{Relative \,formula \,mass}$

$Volume \,of \,gas \,(dm^{3})=number \,of \,moles \,(mol) \times 24$

Then, students must use the correct formula and information to carry out the corresponding calculations. For instance, if a student is reacting nitrogen and hydrogen to make ammonia, then they should use the third formula. Importantly, IGCSE chemistry questions that require mole calculations usually also assess a learner's understanding of chemical equations and how changes in amounts or the type of reactants may affect the rate of reaction.

Sample question 1 & student response

IGCSE Chemistry measuring cylinder example

The measuring cylinder shown contains $0.010 \,mol/dm^{3}$ hydrochloric acid.

a) What volume of hydrochloric acid is in the apparatus?

b) Use your answer in (a) to calculate the amount, in moles, of hydrochloric acid in the apparatus.

Student Response:

a) $44$

b) $44 \times 0.01$

$= 0.44 \,moles$

For part a), the student will only score 1 mark because they read the diagram correctly, but did not state the unit of measurement (cm³). For part b), the student will also only gain 1 mark, since they applied the correct formula, but did not remember to convert the 44cm³ to dm³ by dividing 1000. It is crucial to remember that all mole calculations should be in dm³, and 1 dm³ = 1 litre.

Sample question 2 & student response

A student heats a sample of hydrated copper(II) sulfate crystals, $\ce{CuSO4 * 5H2O}$.

The equation for the reaction that takes place is

$\ce{CuSO4 * 5H2O(s) -> CuSO4(s) + 5H2O(l)}$

Calculate the maximum mass of water that could be collected when a sample of hydrated copper(II) sulfate of mass 2.50 g is heated.
[Mr of $\ce{CuSO4 * 5H2O}$ is 250]

Student Response:

$2.50 \div 250 = 0.01$

$0.01 \times 5 = 0.05$

$\therefore 0.05l \,or \,50g$

This student will score 2 marks because although they calculated the moles of hydrated copper(II) sulfate correctly and multiplied it by 5 (1 mole of hydrated copper(II) sulfate produces 5 moles of water in this reaction), they did not multiply this with the relative formula mass of water.

BartyED and IGCSE chemistry

The IGCSE chemistry curriculum is challenging for even the most adept students. The IGCSE chemistry examination aims to push students’ knowledge and, so, the assistance of an experienced tutor can be greatly beneficial. At BartyED, our team of expert science tutors are positioned to assist all IGCSE students, whether they study chemistry, biology, physics, at all levels. If you think that you or your child could benefit from the guidance of an IGCSE chemistry tutor, reach out today by phone (+852 2882 1017) or email (enquiries@bartyed.com).