AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Students should be able to use the relative formula mass of a substance to…
Full specification wording
Moles (HT only): Students should be able to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa.
The relative formula mass, Mr, of a substance is the sum of the relative atomic masses of all atoms in its formula.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The relative formula mass, Mr, of a substance is the sum of the relative atomic masses of all atoms in its formula. The mass of one mole of a substance in grams is numerically equal to its Mr. To find moles from mass, use amount in moles = mass in g ÷ Mr. To find mass from moles, rearrange to mass in g = amount in moles × Mr. For example, carbon dioxide has Mr = 12 + (2 × 16) = 44, so 22 g of CO₂ is 22 ÷ 44 = 0.50 mol, and 0.25 mol of CO₂ has mass 0.25 × 44 = 11 g. Always include units and give answers to an appropriate number of significant figures. This relationship applies to any substance when its formula is known. This is Higher Tier only content.
What earns the marks
- Calculate relative formula mass by summing relative atomic masses from the formula, including any brackets and multipliers.
- State that the mass of one mole of a substance in grams is numerically equal to its relative formula mass.
- Use the equation amount in moles = mass in g ÷ relative formula mass to calculate moles from a given mass.
- Rearrange the equation to mass in g = amount in moles × relative formula mass to calculate mass from a given number of moles.
- Substitute numerical values correctly and evaluate with consistent units, usually grams and moles.
- Round the final answer to an appropriate number of significant figures and include the correct unit, mol or g.
Common mistakes and how to put them right
Using relative atomic mass instead of relative formula mass for a compound.
Put it right by summing all atoms in the formula, for example Mr of H₂O = 2 × 1 + 16 = 18.
Dividing Mr by mass instead of mass by Mr.
Put it right by checking that the rearrangement gives moles as the subject.
Forgetting to multiply by a subscript or bracket multiplier.
Put it right by expanding the formula carefully, such as Mr of Ca(OH)₂ = 40 + 2 × (16 + 1) = 74.
Exam tips
- Write down the formula and calculate Mr as a separate step before substituting into the mole equation.
- Check the rearrangement by substituting simple numbers, for example 44 g ÷ 44 g mol⁻¹ = 1 mol.
- As this is Higher Tier only content, you must be able to rearrange the mole equation confidently to find mass, moles, or relative formula mass.
Try the lesson
The start of the Moles (HT only): Students should be able to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa. lesson, made from checked teaching notes for this specification point.
Have a go first
The relative formula mass of calcium carbonate, CaCO₃, is 100. What is the number of moles in 250 g of calcium carbonate?
Listen
The relative formula mass, Mr, of a substance is the sum of the relative atomic masses of all atoms in its formula. The mass of one mole of a substance in grams is numerically equal to its Mr. To find moles from mass, use amount in moles = mass in g ÷ Mr. To find mass from moles, rearrange to mass in g = amount in moles × Mr. For example, carbon dioxide has Mr = 12 + (2 × 16) = 44, so 22 g of CO₂ is 22 ÷ 44 = 0.50 mol, and 0.25 mol of CO₂ has mass 0.25 × 44 = 11 g. Always include units and give answers to an appropriate number of significant figures. This relationship applies to any substance when its formula is known. This is Higher Tier only content. A strong answer earns marks for these points. Calculate relative formula mass by summing relative atomic masses from the formula, including any brackets and multipliers. State that the mass of one mole of a substance in grams is numerically equal to its relative formula mass. Use the equation amount in moles = mass in g ÷ relative formula mass to calculate moles from a given mass. Rearrange the equation to mass in g = amount in moles × relative formula mass to calculate mass from a given number of moles. A common slip: Using relative atomic mass instead of relative formula mass for a compound. Instead, summing all atoms in the formula, for example Mr of H₂O = 2 × 1 + 16 = 18. Exam tip: Write down the formula and calculate Mr as a separate step before substituting into the mole equation.
Key words
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Relative formula mass
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Mole
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Amount in moles
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Mass in g
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See every definition
- Relative formula mass
- The sum of the relative atomic masses of all atoms in a substance's formula.
- Mole
- The amount of a substance whose mass in grams is numerically equal to its relative formula mass.
- Amount in moles
- Calculated as mass in g divided by relative formula mass.
- Mass in g
- Calculated by rearranging the equation to amount in moles multiplied by relative formula mass.
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