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# 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 [formula: A formula is a combination of symbols that indicates the chemical composition of a substance. ].

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

ElementRelative atomic mass (Ar)
H1
C12
O16
Na23
Mg24

## Working out Mr

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 Mr of carbon monoxide, CO

• Mr = 12 + 16 = 28

Example 2

• Find the Mr of sodium oxide, Na2O

• Mr = (23 × 2) + 16 = 46 + 16 = 62

Example 3

• Find the Mr of magnesium hydroxide, Mg(OH)2

• Mr = 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 mole of that substance. For example, the Mr 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

Read on if you're taking the higher paper.

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