Converting Between Mass and Moles Flashcards

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What is meant by the molar mass (Mr) of a substance?

It is the mass of 1 mole of that substance in grams. It has the unit g/mol.

Mass (g) = number of moles × molar mass (g/mol).

Divide the mass in grams by the molar mass. Use: moles = mass (g) ÷ molar mass (g/mol).

Multiply the number of moles by the molar mass. Use: mass (g) = moles × molar mass (g/mol).

You must use grams (g), not kilograms.

Read its Ar from the periodic table. For Mg, Ar = 24, so the molar mass is 24 g/mol.

Add the Ar values of the atoms. Mr = (2 × 1) + 16 = 18 g/mol.

Moles = mass ÷ Mr = 10 g ÷ 18 g/mol ≈ 0.56 mol.

Mass = moles × Mr = 0.5 mol × 44 g/mol = 22 g.

It links the mass you can measure on a balance to the amount of substance in moles. This helps you work out how much of a substance to use or make.