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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 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 (A_{r}) |
---|---|

H | 1 |

C | 12 |

O | 16 |

Na | 23 |

Mg | 24 |

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, COM

_{r}= 12 + 16 = 28

**Example 2**

Find the M

_{r}of sodium oxide, Na_{2}OM

_{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)

The relative formula mass of a substance - shown in grams - is called **one mole** of 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.

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