Relative formula mass

Last time

Before we start: last time, in Water of crystallisation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Weighing a formula

A chemist cannot count atoms, but she can add up their masses on paper. Every chemical calculation in the rest of this course starts from one number: the relative formula mass. It is just a sum, and once you can do it, the mole calculations become much easier.

Ar and Mr

The relative atomic mass, Ar, of an element is found in the Periodic Table. For carbon it is twelve, and for oxygen sixteen. The relative formula mass, Mr, of a compound is the sum of the relative atomic masses of all the atoms in its formula. For a simple molecule, it is called the relative molecular mass, and it is worked out in exactly the same way. Relative masses are ratios, so they have no units.

The Ar values to use

You are given a Periodic Table in the exam, but these values come up again and again. Hydrogen one, carbon twelve, nitrogen fourteen, oxygen sixteen. Sodium twenty-three, magnesium twenty-four, aluminium twenty-seven, sulfur thirty-two, chlorine thirty-five point five. Calcium forty, iron fifty-six, copper sixty-four.

The method

First, write down the formula, and list each element in it. Second, count how many atoms of each element there are. The small number after a symbol tells you. Third, multiply the number of atoms by the Ar, for each element. Fourth, add everything up, and check that you have not left an element out.