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You can use information about reacting masses to calculate the formula of a compound [**compound**: *A compound is a substance formed by the chemical union (involving bond formation) of two or more elements.* ]. Here is an example:

- Question
Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?

Use the fact that the A

_{r}of sulfur is 32 and the A_{r}of oxygen is 16.

- Answer
- Find the mass of each element. Conservation of mass tells us that the mass of oxygen = the mass of sulfur oxide - the mass of sulfur.
- The mass of oxygen reacted = 6.4 - 3.2 = 3.2g
- So we have 3.2g of sulfur and 3.2g of oxygen.
- Now divide the mass of each element by its A
_{r}value. - sulfur: 3.2 ÷ 32 = 0.1
- oxygen: 3.2 ÷ 16 = 0.2
- Finally, find the ratio of the elements.
- You can do this by dividing the results by the smallest of the numbers to give you the number of atoms of each element in the compound.
- In this case the smallest value is 0.1, so divide both results by that.
- S = 0.1 ÷ 0.1 = 1
- O = 0.2 ÷ 0.1 = 2
- (If one of the numbers ends in 0.5, multiply all the numbers by 2 - this is because you cannot have half-atoms in a compound.)
- So the ratio of sulfur to oxygen is 1:2

The number of atoms tells you that the formula for sulfur oxide is SO

_{2}

Here is the calculation again in tabular form to help you remember the steps:

step | action | S | O |
---|---|---|---|

1 | find masses | 3.2 | 3.2 |

2 | look up given A_{r} values | 32 | 16 |

3 | divide masses by A_{r} | 0.1 | 0.2 |

4 | find the ratio | 1 | 2 |

Result: the formula for the oxide = SO_{2}

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