Using the Idea of Moles to Find the Formula of a Compound

Finding Empirical Formulae

Using the idea of moles to find the formula of a compound.

The empirical formula of a compound shows the simplest ratio of the elements in a compound. For most of the compounds at GCSE the empirical formula will be the same as the molecular formula. Everybody knows that the formula of water is H2O, in a water molecule there are two hydrogen atoms and one oxygen atom.

So when we want to find out what the formula of a compound is, we have to find out the number of moles of each of the elements that have combined together. In water we would find that two moles of hydrogen combine with one mole of oxygen, which is two moles of hydrogen atoms combine with one mole of oxygen atoms so the simplest ratio is two hydrogen atoms to one oxygen atom.

Where to begin?

When we do an experiment to find the formula of a compound we measure the masses of the elements that react. Sometimes you are given the percentages of the elements that make up the compound, treat them as masses too. Then we turn the masses of each element into moles. We then find the simplest ratio between the numbers of moles and that is the empirical formula.

1. Get the mass of each element, (you may have to calculate it or use a percentage as a mass).
2. Convert the mass of each element into moles of each element using,
3. Find the simplest ratio of the moles of each element by dividing the number of moles of each by the smallest number of moles.
4. Use the mole ratio to write the empirical formula.

Worked Example.

4.8g of magnesium are reacted with 32g of bromine to form magnesium bromide, calculate the empirical formula.

1)Get the masses of each element, Mg-4.8g, Br-32g

2)Find the moles of each one using moles = mass/molar mass.

1. Moles of Mg = 4.8/24 = 0.2 moles
2. Moles of Br = 32/80 = 0.4 moles

3)Find the simplest ratio between the elements by dividing by the lowest number, in this case 0.2 moles

1. Mole ratio of Mg = 0.2/0.2 = 1
2. Mole ratio of Br = 0.4/0.2 = 2

4)Write the ratio into the empirical formula; each mole of Mg is combined with two moles of bromine.

So the formula of magnesium bromide is MgBr2.

An alternative tabular format

Some problems for you; Find the formula of each of the following.

1. Iron oxide, in which 11.2g of iron combine with3.2g of oxygen gas.
2. Carbon fluoride, where 24g of carbon react with 152g of fluorine.
3. Methane, in which 2.4g of carbon combine with 0.8g of hydrogen gas.
4. Copper oxide, where 32g of copper react with 4g of oxygen.
5. Sodium Hydride, in which 6.9g of sodium combine with 0.3g of hydrogen.
6. Sulphur oxide, in which 1g of sulphur makes 2g of sulphur oxide.
7. Lead Bromide, where 2.07g of lead combines with 1.6g of bromine.
8. Iron oxide, in which 5.6g of iron combine with 2.4g of oxygen.
9. Ammonia, where 2.8g of nitrogen combine with 0.6g of hydrogen.
10. Nitrogen oxide, in which 0.7g of nitrogen and 1.6g of oxygen react fully.
11. Lead oxide, 2.07g of lead are used to make 2.23g of the oxide.
12. Sodium nitrate which contains 2.3g of sodium, 1.4g of nitrogen, 4.8g of oxygen

If the question uses percentage composition, treat it as a mass.

1. Calcium carbonate, which is 40% calcium, 12% carbon and 48% oxygen.
2. Magnesium sulphate, which is 26.6% sulphur, 20.2% magnesium and the rest is oxygen
3. An oxide of carbon contains 27.27% carbon. Find its empirical formula?
4. 2.70g of aluminium combines with 10.65g of chlorine. What is the empirical formula of the new substance?
5. Saltpetre is a potassium salt. It is 13.9% nitrogen, 38.6% potassium and 47.5% oxygen. What is its empirical formula?
6. Fluorite is a mineral of calcium and fluorine. If 51.3% is calcium, find the empirical formula.
7. A hydrocarbon has 80% carbon and 20 %hydrogen.
8. Calculate its empirical formula.
9. When the mass of a single molecule of this compound was analysed, it was found to be 30. What must the true (molecular) formula of this substance be?
10. Iron wool was heated in a glass tube, chlorine gas was passed over it and iron chloride formed. The glass tube had a mass of 46.43g, when the iron was added the mass went up to 47.74g. When the reaction was over the mass of the cooled tube and the iron chloride was 50.34g.
11. Find the mass of the iron used.
12. Find the mass of the chlorine added.
13. Find the formula of the iron chloride formed.
14. Write a balanced equation for the formation.