1. Percentages of carbon, hydrogen and oxygen from combustion
Elemental Analysis
Percentages of carbon, hydrogen and oxygen from combustion

2. Combustion Whenever a hydrocarbon burns, CO2 and H2O are formed.
CH4 + O2 g CO2 + H2O
All of the carbon was transferred to carbon dioxide
All of the hydrogen was transferred to water

3. Using mass of products
Knowing how much CO2 and H2O was created, it is then possible to determine the mass of C and H in the hydrocarbon.
From mass, mass % may be found
Knowing %C and %H, the % of O may be determined by subtracting the others from 100%

4. Example
Acetic acid contains only C, H and O. A 4.24 mg sample of acetic acid is completely burned. It gives 6.21 mg of CO2 and 2.54 mg of H2O. What is the mass percentage of each element in acetic acid?
Do stoich to calculate mass of C.
6.21 mg CO2 | 1 mol CO2 | 1 mol C | 12 g C = 1.69mgC
| g CO2 | 1 mol CO2 | 1 mol C
Do stoich to calculate mass of H.
2.54 mg H2O | 1 mol H2O | 2 mol H | 1 g H = 0.285mgH
| g H2O | 1 mol H2O | 1 mol H
Find %.
1.69/4.24 = 39.9% C and 0.285/4.24 = 6.72% H
Therefore 100 – 39.9 – 6.72 = 53.4% O