The relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms shown in the molecular formula of a substance; like Ar, it has no unit.
The relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms shown in the molecular formula of a substance. Like the relative atomic mass, it is a ratio and has no unit. Malay: jisim molekul relatif · Chinese: 相对分子质量.
To find the Mr you simply add up the Ar of every atom in the formula, counting each atom as many times as its subscript shows. It tells you how heavy one molecule of the substance is compared with one-twelfth of a carbon-12 atom, which is exactly the same standard used for a single atom, just applied to a whole molecule.
Example. Water, H2O, has a relative molecular mass of 2(1) + 16 = 18, because it contains two hydrogen atoms of Ar 1 and one oxygen atom of Ar 16. Carbon dioxide, CO2, works out as 12 + 2(16) = 44. The Ar values themselves are read from the Periodic Table provided in the exam.
Confusion to avoid. Use relative molecular mass only for substances that exist as molecules, such as water, carbon dioxide or glucose. For an ionic compound like sodium chloride, which is not made of molecules, the equivalent total is called the relative formula mass instead. The arithmetic is identical, but the exam expects the correct name for the type of substance. Do not confuse Mr with molar mass either: they share the same number, but molar mass carries the unit grams per mole.
In this chapter the relative molecular mass feeds directly into the mole equation, because the molar mass in grams per mole is numerically equal to the Mr. Getting the Mr right is therefore the gateway to every mass-to-mole calculation that follows.
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