AQA · GCSE · Chemistry
Course version: 8462 · 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.
The mole is the chemist's counting unit for particles.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The mole is the chemist's counting unit for particles. For any substance, the relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in its formula. If you take that many grams of the substance, you have exactly one mole of it. For example, water H₂O has Mr = (2 × 1) + 16 = 18, so one mole of water has a mass of 18 g. Carbon dioxide CO₂ has Mr = 12 + (2 × 16) = 44, so one mole of CO₂ has a mass of 44 g. This numerical bridge lets you convert between mass in grams and amount in moles using n = m ÷ Mr, where n is amount in moles, m is mass in grams and Mr is relative formula mass. The same relationship applies to elements, compounds and ions, provided you use the correct formula.
What earns the marks
- State that the relative formula mass Mr is found by adding the relative atomic masses of all atoms shown in the formula.
- Explain that the mass of one mole in grams is numerically equal to Mr, for example 44 g for CO₂ because Mr = 44.
- Use the relationship n = m ÷ Mr correctly, including rearranging to m = n × Mr when mass is required.
- Apply the idea to a range of substances, including simple molecules, ionic compounds and elements, using the correct formula for each.
Common mistakes and how to put them right
Adding atomic masses incorrectly, such as forgetting to multiply by the subscript in CO₂.
Put it right by multiplying each element's Ar by its subscript before adding.
Confusing relative formula mass with the mass of one molecule in grams.
Put it right by stating that Mr is a relative value and the molar mass is that number in grams per mole.
Using the wrong rearrangement of n = m ÷ Mr.
Put it right by checking units and rearranging carefully, for example m = n × Mr.
Exam tips
- Always show the calculation of Mr step by step so the examiner can follow your reasoning.
- Include units throughout, using g for mass, g mol⁻¹ for molar mass and mol for amount.
- Check whether the question asks for mass, moles or Mr, and rearrange the equation before substituting numbers.
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
What is the mass of one mole of water, H₂O, in grams? (Aᵣ: H = 1, O = 16)
Listen
The mole is the chemist's counting unit for particles. For any substance, the relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in its formula. If you take that many grams of the substance, you have exactly one mole of it. For example, water H₂O has Mr = (2 × 1) + 16 = 18, so one mole of water has a mass of 18 g. Carbon dioxide CO₂ has Mr = 12 + (2 × 16) = 44, so one mole of CO₂ has a mass of 44 g. This numerical bridge lets you convert between mass in grams and amount in moles using n = m ÷ Mr, where n is amount in moles, m is mass in grams and Mr is relative formula mass. The same relationship applies to elements, compounds and ions, provided you use the correct formula. A strong answer earns marks for these points. State that the relative formula mass Mr is found by adding the relative atomic masses of all atoms shown in the formula. Explain that the mass of one mole in grams is numerically equal to Mr, for example 44 g for CO₂ because Mr = 44. Use the relationship n = m ÷ Mr correctly, including rearranging to m = n × Mr when mass is required. Apply the idea to a range of substances, including simple molecules, ionic compounds and elements, using the correct formula for each. A common slip: Adding atomic masses incorrectly, such as forgetting to multiply by the subscript in CO₂. Instead, multiplying each element's Ar by its subscript before adding. Exam tip: Always show the calculation of Mr step by step so the examiner can follow your reasoning.
See it
Put it in order
Put these steps in the order they happen.
- 1. Relative formula mass Mr
- 2. Add relative atomic masses of all atoms in the formula
- 3. Formula of substance
- 4. Rearrange to m = n × Mr when mass is required
- 5. n = m ÷ Mr
- 6. Mass of one mole in grams is numerically equal to Mr
Key words
Say what each word means before you turn the card.
Mole concept
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Relative formula mass
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Mass–mole conversions
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See every definition
- Mole concept
- The mole is the chemist's counting unit for particles, and one mole of a substance has a mass in grams numerically equal to its relative formula mass.
- Relative formula mass
- The sum of the relative atomic masses of all atoms in a substance's formula, found by multiplying each element's relative atomic mass by its subscript before adding.
- Mass–mole conversions
- The numerical bridge that lets you convert between mass in grams and amount in moles using n = m ÷ Mr, rearranging to m = n × Mr when mass is required.
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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