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

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What calculation is used to determine the moles of a gas in a given mass?

Moles = mass x Mᵣ
Moles = mass / Mᵣ
Moles = Mᵣ / mass
Moles = mass + Mᵣ

The correct answer is: Moles = mass / Mᵣ

To determine the moles of a gas from a given mass, the calculation used is moles equals mass divided by the molar mass (Mᵣ). This relationship is grounded in the concept that the amount of substance in moles is directly related to the mass of the substance and its molar mass. When you divide the mass of the substance by its molar mass, you are converting the mass of the gas from grams into moles, which is the standard unit of measurement in chemistry for the amount of a substance. The molar mass is the mass of one mole of a substance expressed in grams per mole (g/mol). By using this formula, you can accurately determine how many moles of gas are present in a specific mass. This calculation is fundamental in stoichiometry, allowing chemists to relate quantities of different substances involved in chemical reactions, making it essential for both theoretical and practical applications in chemistry.