Calculating the Relative Molecular Mass (RMM) of Compounds

The Relative Molecular Mass of a compound is the sum of the masses of all the atoms present in the molecule. It is often shortened to RMM. The RMM is used in many sorts of calculations in chemistry, and so you must be able to calculate it to answer all the other calculations you might meet. This is not particularly difficult, as long as you can do simple arithmatic and know the relative atomic masses of the elements, sometimes called the RAM of an element. The following examples show you how to do this. If you already know how to do these calculations, move on to the RMM Tests page for some practice. But if you are a bit unsure, here are some examples for you. We will use the relative atomic masses given below. It might help if you have some paper and a pen handy, and use a calculator.

Relative Atomic Masses
Element	Symbol	RAM
Carbon	C	12
Hydrogen	H	1
Oxygen	O	16
Sodium	Na	23
Sulphur	S	32
Chlorine	Cl	35.5

Example One

What is the Relative Molecular Mass of sodium sulphate?

Sodium sulphate has the formula Na₂SO₄ - that's two sodiums, one sulphur, and four oxygens.

Na	2 x 23	= 46
S	32	= 32
O	4 x 16	= 64
Total RMM		142

So we just add up the masses of all these atoms to get the total mass - the relative molecular mass 142 in this example.

Example Two

What is the RMM of ether?

Ether has the formula CH₃CH₂OCH₂CH₃ . There are two ways to calculate the RMM here.

CH3 12 + (3 x 1) = 15 CH2 12 + (2 x 1) = 14 O 16 = 16 CH2 12 + (2 x 1) = 14 CH3 12 + (3 x 1) = 15 Total RMM 74

OR

C 4 x 12 = 48 H 10 x 1 = 10 O 16 = 16 Total RMM 74

The method on the left takes each small group in the molecule and works out its mass first; the method on the right counts up all the atoms of each type separately. Which method you use is up to you, as you get the same answer either way.

Example Three

Calculate the relative molecular mass of ammonium sulphate.

Ammonium sulphate has the chemical formula (NH₄)₂SO₄.

(NH4)2 2 x (14 + (4 x 1)) = 36 S 32 = 32 O4 4 x 16 = 64 Total RMM 132

OR

N 2 x 14 = 28 H 8 x 1 = 8 S 32 = 32 O 4 x 16 = 64 Total RMM 132

Here we have multiplied the mass of a single NH₄ by two in the calculation on the left because there are two such groups in the molecule. In the calculation on the right we have multiplied the number of nitrogens and hydrogens by two as well.

Example Four

What is the RMM of (CH₂Cl)₂CHCH(CHCl₂)₂?

(CH2Cl)2 2 x (12 + (2 x 1) + 35.5) = 99 CH 12 + 1 = 13 CH 12 + 1 = 13 (CHCl2)2 2 x (12 + 1 + (2 x 35.5)) = 168 Total RMM 293

OR

C 6 x 12 = 72 H 8 x 1 = 8 Cl 6 x 35.5 = 213 Total RMM 293

Notice that it doesn't matter how complicated the formula is, or whether you know what the compound is or have never seen it before in your life, you just work out the answer step by step. Then you check it again, in case you hit the wrong key on the calculator, or got the brackets wrong. Everyone makes the odd arithmatic slip, so always check your answer. And do use brackets when you write down calculations like this, it prevents little mistakes ruining your good work. It only takes a second longer to put them in place, but it helps you get the marks.

Look carefully at each example to see exactly how it has been done. Practise with compounds whose mass is given in a textbook. Because if the first time you try this for yourself is in an exam, you will have a problem.

Here are a couple to check for yourself - H₂SO₄ = 98, and Na₂CO₃ = 106.