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:
2SO2 + O2 --> 2SO3
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