GCSE Chemistry revision

Amounts of substances in equations

Use of amount of substance in relation to masses of pure substances

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To find the mass of 3 moles of CO₂ (Mr = 44): 3 × 44 = 132 g You can also calculate the mass of a specific element within a compound by multiplying the number of moles by the Ar of that element.
For example, the mass of carbon in 3 moles of CO₂: 3 × 12 = 36 g of carbon By subtracting the mass of carbon (36 g) from the total mass (132 g), you can deduce there are 96 g of oxygen in the sample.

1. Calculating Mass from Moles AQA

To find the mass of 3 moles of CO₂ (Mr = 44): 3 × 44 = 132 g You can also calculate the mass of a specific element within a compound by multiplying the number of moles by the Ar of that element.
For example, the mass of carbon in 3 moles of CO₂: 3 × 12 = 36 g of carbon By subtracting the mass of carbon (36 g) from the total mass (132 g), you can deduce there are 96 g of oxygen in the sample.

The Mr of a compound is found by adding together the Ar values of all atoms present in its formula.
For CO₂: Mr = 12 + (2 × 16) = 44 For NH₃: Mr = 14 + (3 × 1) = 17