AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Amounts of substances in equations (HT only): Mg + 2HCl → MgCl₂ + H₂
This balanced equation is a quantitative recipe for the reaction between magnesium and hydrochloric acid.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This balanced equation is a quantitative recipe for the reaction between magnesium and hydrochloric acid. The large numbers in front of formulae are stoichiometric coefficients: they tell you the ratio in which particles react and are produced. One Mg atom reacts with two HCl molecules, giving one MgCl₂ formula unit and one H₂ molecule. The small subscript ₂ in H₂ shows atoms within a particle, not amounts that can be changed. To use the equation, convert masses to moles, apply the coefficients as a ratio, then convert back.
For example: 2.4 g Mg is 0.10 mol, so it needs 0.20 mol HCl and forms 0.10 mol H₂, which has a mass of 0.20 g.
What earns the marks
- State that coefficients give the simplest whole-number mole ratio: 1 mol Mg : 2 mol HCl : 1 mol MgCl₂ : 1 mol H₂.
- Explain that subscripts inside a formula, such as the ₂ in H₂, are fixed by the substance and cannot be altered to balance an equation.
- Convert a given mass to moles using moles = mass ÷ relative formula mass, then multiply by the coefficient ratio to find moles of another substance.
- Calculate a reacting mass from the mole ratio, for example 0.10 mol H₂ has a mass of 0.20 g.
- Recognise that the equation alone does not give rate or yield; it gives the stoichiometric amounts that react completely.
Common mistakes and how to put them right
Reading 2HCl as two hydrogen atoms and two chlorine atoms in one particle.
Put it right by treating 2HCl as two separate HCl molecules, each with one H and one Cl.
Changing the subscript in MgCl₂ to MgCl to balance atoms.
Put it right by placing a coefficient before a formula instead of altering the formula itself.
Assuming equal masses react in a 1:1 ratio.
Put it right by converting each mass to moles first, because the mole ratio is 1:2:1:1.
Exam tips
- Underline the coefficients before starting a calculation so you apply the 1:2:1:1 ratio correctly.
- Show the moles of each substance in a clear sequence, then convert only the substance asked for into mass.
- Check that your final answer has the correct unit and a sensible size; a small mass of magnesium should not produce an enormous mass of hydrogen.
Try the lesson
The start of the Amounts of substances in equations (HT only): Mg + 2HCl → MgCl₂ + H₂ lesson, made from checked teaching notes for this specification point.
Have a go first
The balanced equation for a reaction is: Mg + 2HCl → MgCl₂ + H₂. According to this equation, how many moles of hydrochloric acid are required to react exactly with one mole of magnesium?
Listen
This balanced equation is a quantitative recipe for the reaction between magnesium and hydrochloric acid. The large numbers in front of formulae are stoichiometric coefficients: they tell you the ratio in which particles react and are produced. One Mg atom reacts with two HCl molecules, giving one MgCl₂ formula unit and one H₂ molecule. The small subscript ₂ in H₂ shows atoms within a particle, not amounts that can be changed. To use the equation, convert masses to moles, apply the coefficients as a ratio, then convert back. For example, 2.4 g Mg is 0.10 mol, so it needs 0.20 mol HCl and forms 0.10 mol H₂, which has a mass of 0.20 g. A strong answer earns marks for these points. State that coefficients give the simplest whole-number mole ratio: 1 mol Mg : 2 mol HCl : 1 mol MgCl₂ : 1 mol H₂. Explain that subscripts inside a formula, such as the ₂ in H₂, are fixed by the substance and cannot be altered to balance an equation. Convert a given mass to moles using moles = mass ÷ relative formula mass, then multiply by the coefficient ratio to find moles of another substance. Calculate a reacting mass from the mole ratio, for example 0.10 mol H₂ has a mass of 0.20 g. A common slip: Reading 2HCl as two hydrogen atoms and two chlorine atoms in one particle. Instead, treating 2HCl as two separate HCl molecules, each with one H and one Cl. Exam tip: Underline the coefficients before starting a calculation so you apply the 1:2:1:1 ratio correctly.
See it
Key words
Say what each word means before you turn the card.
Stoichiometric coefficients
Try answering aloud or in your head before turning the card.
Subscripts
Try answering aloud or in your head before turning the card.
Stoichiometric amounts
Try answering aloud or in your head before turning the card.
See every definition
- Stoichiometric coefficients
- The large numbers in front of formulae that tell you the ratio in which particles react and are produced.
- Subscripts
- The small numbers inside a formula, such as the ₂ in H₂, which show atoms within a particle and are fixed by the substance.
- Stoichiometric amounts
- The amounts that react completely, which the equation alone gives, without rate or yield.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Amounts of substances in equations (HT only): Mg + 2HCl → MgCl₂ + H₂
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge