# Formulae of Compounds

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## Formulae of Compounds

**The formula of water is H _{2}O.** This is because 1 atom of oxygen combines with

**2**atoms of hydrogen.

Alternatively, we can say that **1 mole of oxygen **atoms combines with **2 moles of hydrogen** atoms to form 1 mole of water molecules.

Moles can be changed to grams; therefore we can say **16 grams of oxygen** combine with **2 grams of hydrogen** to form **18 grams of water.**

The formula of carbon dioxide is CO_{2}. This is because **1 **atom of carbon combines with **2 **atoms of oxygen. Therefore, 12 grams of carbon combine with 32 grams of oxygen to form 44 grams of carbon dioxide.

Alternatively, **1 mole of carbon atoms** combine with **2 moles of oxygen atoms **to produce **1 mole of carbon dioxide.**

**To find the formula of a compound**

- Start with the number of grams that combine
- Change the grams to moles
- This gives you the ratio which they combine
- So now you know the formula

A formula obtained in this way is called the **Empirical Formula. **

**Example:**

- 32 grams of sulphur react with 32 grams of oxygen
- 1 mole of sulphur reacts with 2 moles of oxygen
- Ratio of 1:2
- Formula SO
_{2}- sulphur dioxide

**The Reaction between Magnesium and Oxygen**

The apparatus below is used to calculate the mass of magnesium oxide, but firstly you must know the masses of the elements that combine.

**Method:**

- Weigh the mass of the crucible and lid
- Add a coil of magnesium ribbon and reweigh.
- Heat crucible strongly, lifting lid occasionally to allow oxygen in.
- When burning is complete allow the apparatus to cool.
- With the lid on reweigh the crucible and its contents.

**Results:**

mass of magnesium oxide - mass of magnesium = mass of oxygen

4.0g - 2.4g = 1.6g

**2.4g of magnesium = 0.1 moles **

**1.6g of oxygen = 0.1 moles**

Therefore the ratio of magnesium to oxygen used is 1:1

**Conclusion:**

The formula of magnesium oxide is MgO.