Molar mass

The molar mass (M) is the mass of one mole of a substance. It is numerically equal to the relative atomic mass or relative molecular mass, but it is expressed in grams per mole (g mol⁻¹). Malay: jisim molar · Chinese: 摩尔质量.

Molar mass is the practical link between the amount in moles and a mass you can weigh out on a balance. Its great convenience is that you do not have to calculate it separately: you simply take the relative atomic or molecular mass and attach the unit grams per mole. This is why the relative masses in this chapter matter so much.

Example. Because water has a relative molecular mass of 18, its molar mass is 18 g mol⁻¹, so one mole of water weighs 18 g. Oxygen gas, O₂, has a molar mass of 32 g mol⁻¹. To find how many moles are in a given mass, you use the mole equation, dividing the mass by the molar mass.

Confusion to avoid. Do not confuse molar mass with relative molecular mass. They share the same number, but molar mass carries the unit g mol⁻¹ while the relative molecular mass has no unit at all. Losing the unit, or writing the unit on a relative mass, is a common way to drop a mark. Remember too that the molar mass of an element as single atoms differs from that of its molecules, so oxygen atoms are 16 g mol⁻¹ but oxygen gas O₂ is 32 g mol⁻¹.

In this chapter the molar mass is the value M in the equation n = m ÷ M, which converts between mass and moles. It is therefore one of the most-used quantities in every stoichiometry problem across SPM Chemistry.