Relative formula mass

For your exam, you will need to know what relative formula mass is. You should also be able to work out the relative formula mass of a substance when given its formula.

The symbol for relative formula mass is M_r. The symbol for relative atomic mass is A_r. You will be given any A_r values you need in the examination. The table shows some of these values:

 

Element	Relative atomic mass (Ar)
H	1
C	12
O	16
Na	23
Mg	24

Working out M_r

To find the relative formula mass of a substance, you just add together the relative atomic mass values for all the atoms in its formula. Here are three examples:

Example 1

• Find the M_r of carbon monoxide, CO
• M_r = 12 + 16 = 28

Example 2

• Find the M_r of sodium oxide, Na₂O
• M_r = (23 × 2) + 16 = 46 + 16 = 62

Example 3

• Find the M_r of magnesium hydroxide, Mg(OH)₂
• M_r = 24 + 2 × (16+1) = 24 + 34 = 58
• (Remember that there are two of each atom inside the brackets)

Moles

The relative formula mass of a substance - shown in grams - is called one moleof that substance. For example, the M_r of carbon monoxide (CO) is 28. This means that one mole of carbon monoxide has a mass of 28 g. You should be able to see that:

• 14 g of carbon monoxide contains 14 ÷ 28 = 0.5 moles
• 56 g of carbon monoxide contains 56 ÷ 28 = 2 moles

Now try a Test Bite.

Read on if you're taking the higher paper.