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# Relative formula mass

For your exam, you will need to know what relative formula mass is - and be able to work out the relative formula mass of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] when given its formula.

To find the relative formula mass (Mr) of a compound, you just add together the Ar values for all the atoms in its formula.

## Examples

• Example 1: Find the Mr of carbon monoxide (CO).
• The Ar of carbon is 12 and the Ar of oxygen is 16.
• So the Mr of carbon monoxide is 12 + 16 = 28.
• Example 2:Find the Mr of sodium oxide (Na2O).
• The Ar of sodium is 23 and the Ar of oxygen is 16.
• So the Mr of sodium oxide is (23 x 2) + 16 = 62.

The relative formula mass of a substance, shown in grams, is called one mole of that substance. So one mole of carbon monoxide has a mass of 28g, and one mole of sodium oxide has a mass of 62g.

The table shows some more examples of relative formula mass calculations, using the relative atomic mass values given at the bottom of the page.

## Relative formula mass calculations

CompoundFormulaCalculationMr
WaterH2O1 + 1 + 16 =18
Sodium hydroxideNaOH23 + 16 + 1 = 40
Magnesium hydroxideMg(OH)224 + 16 + 16 + 1 + 1 = (remember that there are two of each atom inside the brackets)58
• Ar of H = 1
• Ar of O = 16
• Ar of Na = 23
• Ar of Mg = 24

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