International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

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Which formula represents the mass of a substance based on moles and relative formula mass?

mass = amount (in moles) + Mᵣ
mass = amount (in moles) / Mᵣ
mass = amount (in moles) x Mᵣ
mass = amount (in moles) - Mᵣ

The correct answer is: mass = amount (in moles) x Mᵣ

The formula that accurately represents the relationship between the mass of a substance, the amount in moles, and the relative formula mass (Mᵣ) is mass = amount (in moles) x Mᵣ. This expression is rooted in the concept of molar mass, which defines how many grams of a substance correspond to a mole of that substance. When you multiply the number of moles by the relative formula mass, you arrive at the mass of the substance in grams. The relative formula mass is a weighted average calculated based on the masses of the constituent atoms, and it tells you how much one mole of a given compound weighs. For example, if you have one mole of water (H₂O), and knowing that the relative formula mass of water is approximately 18 g/mol, you can use this formula to find that the mass of water is 1 mole x 18 g/mol = 18 grams. This consistent relationship is fundamental in stoichiometry, where converting between moles and mass is frequently required in chemical calculations and reactions.