1. Percentage Percentage means ‘out of 100’
To calculate the percentage of substance A in substance B, divide A by B and multiply by 100
In Chemistry we need to know the percentage an element makes up of a compound
We calculate this by
(Element Mass/Compound Mass) x 100

2. The percentage of Mg in MgCl2 is calculated by
=25.26%

3. Empirical Formula
An empirical formula shows the ratios of all the elements in that compound
It does not show the relationship between atoms within the compound (how they bond)
Eg. Hydrogen Peroxide has the formula H2O2 and the empirical formula of HO
Glucose has the formula C6H12O6 but the empirical formula of CH2O

4. Calculating Empirical Formula from percentage composition
Assume you have 100g of a sample and that the percentages of each element represent grams of that element
Calculate the number of moles of each element present
Simplify the ratio of moles by dividing the number of moles by the smallest number of moles
Adjust the ratios to whole numbers
Write the empirical formula of the compound

5. Calculating Empirical Formula from percentage composition
What is the empirical formula of a compound containing 80% carbon and 20% hydrogen?

6. 1. Assume we have 100g of the compound 80g Carbon and 20g Hydrogen 2
1. Assume we have 100g of the compound 80g Carbon and 20g Hydrogen 2. Calculate the moles of each

7. 3. Find the simplest ratio of the amounts of each element by dividing the number of moles of each element by the smallest number of moles 4. Adjust to whole numbers 5. Write the empirical formula of the compound 1xC and 3xH CH3

8. Molecular formula from Empirical formula
The molecular formula of a compound will be (empirical formula)x We can figure out what x is if we are given a mass for the compound. To do this we divide the relative molecular mass of the compound by the molecular mass of the empirical formula

9. What is the molecular formula of a compound whose empirical formula is CH2 and molar mass is 98g mol-1