AQA · GCSE · Chemistry
Course version: 8462 · 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 mole is the amount unit linking mass to particles.
Checked against the AQA 8462 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 mole is the amount unit linking mass to particles. One mole of a substance has a mass in grams equal to its relative formula mass, Mr. For a pure substance, amount in moles n = mass m ÷ Mr, and mass m = n × Mr. For example, carbon dioxide CO₂ has Mr = 12 + (2 × 16) = 44, so 88 g contains 88 ÷ 44 = 2 mol, and 0.25 mol has mass 0.25 × 44 = 11 g. For a monatomic element, relative atomic mass Ar is used; for a compound or molecular element (like O₂), add Ar values for every atom in the formula. Rearranging the same relationship gives both directions. Always include units: g for mass and mol for amount. Note that Mr is dimensionless, whereas molar mass has units of g mol⁻¹.
What earns the marks
- State the relationship amount in moles = mass ÷ relative formula mass and rearrange it to mass = moles × relative formula mass.
- Calculate Mr correctly by adding relative atomic masses for every atom shown in the formula, including brackets and any multipliers.
- Substitute values with consistent units, showing the division or multiplication before giving the answer.
- Give the final amount in mol or the final mass in g, with a sensible number of significant figures and correct unit.
- For a reverse calculation, identify the known quantity first, then choose the matching rearrangement rather than dividing when multiplication is required.
Common mistakes and how to put them right
Using relative atomic mass of the first element only for a compound.
Put it right by summing every atom, for example Mr of H₂O is 18, not 1 or 16.
Multiplying when the question asks for moles from mass.
Put it right by dividing mass by Mr for that direction.
Omitting units or writing mol g⁻¹.
Put it right by giving mol for amount and g for mass, remembering that Mr is dimensionless.
Exam tips
- Write the formula triangle or equation n = m ÷ Mr at the start so the rearrangement is visible to you and to the examiner.
- Check whether the substance is a monatomic element (using Ar) or a compound/molecular element (using Mr), and show the addition that gives Mr.
- Estimate first: 88 g of CO₂ should be about 2 mol because Mr is about 44, so a wildly different answer signals an error.
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
Calculate the mass of 0.50 mol of calcium carbonate, CaCO₃. (Mr of CaCO₃ = 100)
Listen
The mole is the amount unit linking mass to particles. One mole of a substance has a mass in grams equal to its relative formula mass, Mr. For a pure substance, amount in moles n = mass m ÷ Mr, and mass m = n × Mr. For example, carbon dioxide CO₂ has Mr = 12 + (2 × 16) = 44, so 88 g contains 88 ÷ 44 = 2 mol, and 0.25 mol has mass 0.25 × 44 = 11 g. For a monatomic element, relative atomic mass Ar is used; for a compound or molecular element (like O₂), add Ar values for every atom in the formula. Rearranging the same relationship gives both directions. Always include units: g for mass and mol for amount. Note that Mr is dimensionless, whereas molar mass has units of g mol⁻¹. A strong answer earns marks for these points. State the relationship amount in moles = mass ÷ relative formula mass and rearrange it to mass = moles × relative formula mass. Calculate Mr correctly by adding relative atomic masses for every atom shown in the formula, including brackets and any multipliers. Substitute values with consistent units, showing the division or multiplication before giving the answer. Give the final amount in mol or the final mass in g, with a sensible number of significant figures and correct unit. A common slip: Using relative atomic mass of the first element only for a compound. Instead, summing every atom, for example Mr of H₂O is 18, not 1 or 16. Exam tip: Write the formula triangle or equation n = m ÷ Mr at the start so the rearrangement is visible to you and to the examiner.
Key words
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Mole as the amount unit
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Relative formula mass from relative atomic masses
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Mass–amount relationship
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Rearranging equations and unit consistency
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See every definition
- Mole as the amount unit
- The amount unit linking mass to particles, where one mole of a substance has a mass in grams equal to its relative formula mass, Mr.
- Relative formula mass from relative atomic masses
- Found by adding relative atomic masses for every atom in the formula; for a monatomic element Ar is used, and for a compound or molecular element the Ar values are added for every atom.
- Mass–amount relationship
- For a pure substance, amount in moles n = mass m ÷ Mr, and rearranging the same relationship gives mass m = n × Mr.
- Rearranging equations and unit consistency
- Rearranging the same relationship gives both directions, and units must always be included: g for mass and mol for amount, with Mr dimensionless whereas molar mass has units of g mol⁻¹.
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