In this I/GCSE Chemistry blog post, we will be continuing on with the main equations used for IGCSE Chemistry.

**Empirical Formula **

To find the empirical formula from a molecular formula, just cancel down the numbers of atoms to their simplest form. E.g. H2O2 would have an empirical formula of HO.

To find the empirical formula of a substance when given the mass of the elements:

**Find the number of moles of each element (mass/mass of 1 mole)**- Divide the results of this by the smallest one (so if there are 2 elements and one has 1 mole but the other has 2 moles, you divide by 1)
**These numbers are the numbers of atoms present in the empirical formula.**

**Percentage Composition **

To find the percentage of a substance made up by one element:

**Mass of the element you want to calculate / Molecular mass of entire molecule * 100**

**Calculating Masses of Substances from an Equation**

E.g. Calculate the mass of oxygen which will react exactly with 0.64g sulfur dioxide according to the equation:

**2**SO2 + O2 --> **2**SO3

1. **MAKE SURE THE EQUATION IS BALANCED BEFORE BEGINNING**

2. **Work out the number of moles of sulfur dioxide.** (0.64 / 32 + 16 + 16 = 0.01 mole)

3. **Find the reaction ratio from the equation.**

(2 moles of sulfur dioxide react with 1 mole of oxygen so the ratio is 2:1)

4. **Use the ratio to find the number of moles of oxygen needed.** (0.01 / 2 = 0.005 moles of oxygen)

5. **Find the mass of 0.005 moles of oxygen.** (0.005 * (16 + 16) = 0.16g)

ANSWER = 0.16g oxygen will react with 0.64g sulfur dioxide.

That's all!

References:

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.aplustopper.com%2Fempirical-and-molecular-formula%2F&psig=AOvVaw1ckLoffxmkdfAMVPCuvP2q&ust=1628045076358000&source=images&cd=vfe&ved=0CAsQjRxqFwoTCOCoit3qk_ICFQAAAAAdAAAAABAD

https://www.google.com/url?sa=i&url=https%3A%2F%2Figcse-chemistry-2017.blogspot.com%2F2017%2F07%2F128-understand-how-to-carry-out.html&psig=AOvVaw0Q-GhJdcdEb0BnmQnZmRAs&ust=1628045127527000&source=images&cd=vfe&ved=0CAsQjRxqFwoTCIDo7fDqk_ICFQAAAAAdAAAAABAD