AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to: calculate the masses of substances shown in a…
Full specification wording
Amounts of substances in equations (HT only): Students should be able to: calculate the masses of substances shown in a balanced symbol equation calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.
This skill has two parts.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 2
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This skill has two parts. First, from a balanced equation alone, calculate the mass of each substance shown by treating the balancing numbers as moles and multiplying each by its Mr. For 2H₂ + O₂ → 2H₂O, 2 mol H₂ has mass 4 g, 1 mol O₂ has mass 32 g and 2 mol H₂O has mass 36 g, so 4 g + 32 g = 36 g, confirming conservation of mass. Second, when a mass of one reactant or product is given, convert it to moles, apply the mole ratio from the equation, then convert to the mass of the required substance. For example, if 8 g of H₂ reacts, n(H₂) = 8 ÷ 2 = 4 mol, so n(H₂O) = 4 mol and m(H₂O) = 4 × 18 = 72 g. Always use the Mr of the substance being converted.
What earns the marks
- Calculate the mass of each substance in an equation by multiplying its balancing number by its Mr.
- Show that the total mass of reactants equals the total mass of products for a fully balanced equation.
- For a given mass, convert to moles using the correct Mr before applying the mole ratio.
- Apply the balancing-number ratio correctly and convert the result to mass using the Mr of the required substance.
- Give answers with correct units and appropriate significant figures.
Common mistakes and how to put them right
Treating balancing numbers as masses.
Put it right by multiplying each balancing number by that substance's Mr.
Forgetting to convert the given mass to moles before using the ratio.
Put it right by starting with n = m ÷ Mr.
Using the Mr of the known substance for the required substance.
Put it right by calculating the required substance's Mr separately.
Exam tips
- Label each substance in the equation with its moles and mass to organise the calculation.
- For the first part, check that reactant masses sum to product masses; this catches arithmetic slips.
- For the second part, write the three stages clearly: moles known, ratio, moles required, then mass required.
Try the lesson
The start of the Amounts of substances in equations (HT only): Students should be able to: calculate the masses of substances shown in a balanced symbol equation calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product. lesson, made from checked teaching notes for this specification point.
Have a go first
In the reaction 2H₂ + O₂ → 2H₂O, calculate the mass of water formed when 4 g of hydrogen reacts completely with oxygen. (Mr: H₂ = 2, H₂O = 18)
Listen
This skill has two parts. First, from a balanced equation alone, calculate the mass of each substance shown by treating the balancing numbers as moles and multiplying each by its Mr. For 2H₂ + O₂ → 2H₂O, 2 mol H₂ has mass 4 g, 1 mol O₂ has mass 32 g and 2 mol H₂O has mass 36 g, so 4 g + 32 g = 36 g, confirming conservation of mass. Second, when a mass of one reactant or product is given, convert it to moles, apply the mole ratio from the equation, then convert to the mass of the required substance. For example, if 8 g of H₂ reacts, n(H₂) = 8 ÷ 2 = 4 mol, so n(H₂O) = 4 mol and m(H₂O) = 4 × 18 = 72 g. Always use the Mr of the substance being converted. A strong answer earns marks for these points. Calculate the mass of each substance in an equation by multiplying its balancing number by its Mr. Show that the total mass of reactants equals the total mass of products for a fully balanced equation. For a given mass, convert to moles using the correct Mr before applying the mole ratio. Apply the balancing-number ratio correctly and convert the result to mass using the Mr of the required substance. A common slip: Treating balancing numbers as masses. Instead, multiplying each balancing number by that substance's Mr. Exam tip: Label each substance in the equation with its moles and mass to organise the calculation.
See it
Put it in order
Put these steps in the order they happen.
- 1. Apply mole ratio from balancing numbers
- 2. Convert to moles: n = m ÷ Mr
- 3. Given mass of a substance
- 4. Answer with correct units
- 5. Convert to mass using Mr of required substance
- 6. Moles of required substance
Key words
Say what each word means before you turn the card.
Conservation of mass
Try answering aloud or in your head before turning the card.
Mole ratios
Try answering aloud or in your head before turning the card.
Mass relationships in equations
Try answering aloud or in your head before turning the card.
See every definition
- Conservation of mass
- The total mass of reactants equals the total mass of products for a fully balanced equation, as when 4 g plus 32 g gives 36 g.
- Mole ratios
- The balancing-number ratio from the equation, applied after converting a given mass to moles, to find the moles of the required substance.
- Mass relationships in equations
- Calculating the mass of each substance by multiplying its balancing number by its Mr, or by converting a given mass to moles and back to mass.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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