AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The mass of one mole of a substance in grams is numerically equal to its…
Full specification wording
Moles (HT only): The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
This Higher Tier only statement links the counting unit, the mole, to mass in grams.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier only statement links the counting unit, the mole, to mass in grams. The relative formula mass, Mr, of a substance is found by adding the relative atomic masses of all atoms in its formula. For water, H₂O, Mr = (2 × 1) + 16 = 18, so one mole of water has a mass of 18 g. For carbon dioxide, CO₂, Mr = 12 + (2 × 16) = 44, so one mole of carbon dioxide has a mass of 44 g. The numerical value of Mr equals the mass in grams of one mole. This lets you convert between mass and moles using the relationship: amount in moles = mass in grams ÷ molar mass. Always calculate Mr from the formula first, then substitute carefully with units to ensure accurate conversions.
What earns the marks
- Calculates relative formula mass by adding relative atomic masses from the formula, for example Mr of CO₂ = 12 + 2 × 16 = 44.
- States that the mass of one mole in grams is numerically equal to the relative formula mass, so 1 mol CO₂ has mass 44 g.
- Uses molar mass to convert between mass and amount in moles.
- Applies the relationship amount in moles = mass in grams ÷ molar mass to a given substance.
- Handles formulae with brackets or coefficients correctly when finding Mr, for example Ca(OH)₂ = 40 + 2 × (16 + 1) = 74.
Common mistakes and how to put them right
Forgetting to multiply by the number of atoms in the formula, such as using 16 for CO₂ instead of 32 for the two oxygen atoms.
Put it right by counting every atom in the formula.
Mixing up the conversion direction, dividing when mass should be multiplied by molar mass.
Put it right by checking units so that grams ÷ g mol⁻¹ gives mol.
Writing the molar mass without units or with the wrong unit.
Put it right by giving g/mol for molar mass and g for mass.
Exam tips
- Show the addition for Mr line by line so the examiner can see how each relative atomic mass was used.
- Write the conversion equation before substituting numbers, then include units in the final answer.
- As this is Higher Tier only, be prepared to use this conversion as part of a larger reacting mass calculation.
Try the lesson
The start of the Moles (HT only): The mass of one mole of a substance in grams is numerically equal to its relative formula mass. lesson, made from checked teaching notes for this specification point.
Have a go first
The relative formula mass of water, H₂O, is 18. What is the mass of one mole of water in grams?
Listen
This Higher Tier only statement links the counting unit, the mole, to mass in grams. The relative formula mass, Mr, of a substance is found by adding the relative atomic masses of all atoms in its formula. For water, H₂O, Mr = (2 × 1) + 16 = 18, so one mole of water has a mass of 18 g. For carbon dioxide, CO₂, Mr = 12 + (2 × 16) = 44, so one mole of carbon dioxide has a mass of 44 g. The numerical value of Mr equals the mass in grams of one mole. This lets you convert between mass and moles using the relationship: amount in moles = mass in grams ÷ molar mass. Always calculate Mr from the formula first, then substitute carefully with units to ensure accurate conversions. A strong answer earns marks for these points. Calculates relative formula mass by adding relative atomic masses from the formula, for example Mr of CO₂ = 12 + 2 × 16 = 44. States that the mass of one mole in grams is numerically equal to the relative formula mass, so 1 mol CO₂ has mass 44 g. Uses molar mass to convert between mass and amount in moles. Applies the relationship amount in moles = mass in grams ÷ molar mass to a given substance. A common slip: Forgetting to multiply by the number of atoms in the formula, such as using 16 for CO₂ instead of 32 for the two oxygen atoms. Instead, counting every atom in the formula. Exam tip: Show the addition for Mr line by line so the examiner can see how each relative atomic mass was used.
See it
Put it in order
Put these steps in the order they happen.
- 1. Convert between mass and amount in moles
- 2. amount in moles = mass in grams ÷ molar mass
- 3. Relative formula mass, Mr
- 4. Molar mass
- 5. Formula of substance
- 6. Add relative atomic masses of all atoms in the formula
- 7. Mass of one mole in grams is numerically equal to Mr
Key words
Say what each word means before you turn the card.
Relative formula mass
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Molar mass and the mole
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Mass–mole conversion
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Chemical formulae and relative atomic masses
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See every definition
- Relative formula mass
- Found by adding the relative atomic masses of all atoms in the formula of a substance.
- Molar mass and the mole
- The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
- Mass–mole conversion
- Amount in moles equals mass in grams divided by molar mass, which lets you convert between mass and moles.
- Chemical formulae and relative atomic masses
- The relative atomic masses of all atoms in the formula are added to find the relative formula 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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