Estimation of Oxygen

Direct Method

1. Decompose the given mass of organic compound, say m g, by heating in stream of nitrogen gas. $$ ce{compound ->[heat, N2] O2 + other gases $$
2. Passing the mixture of product gases over red-hot coke. All the oxygen present in the mixture converts into carbon monoxide. $$ ce{2C + O2->[1373 K] 2CO} …….. (A) $$
3. Passing the mixture containing CO through warm Iodine pentoxide (ce{I2O5}). Carbon monoxide converts into carbon dioxide and produces iodine. $$ ce{5CO + I2O5-> I2 + 5CO2} …….. (B) $$
4. Add the two equations, A and B, after multiplying (A) by 5 and (B) by 2, we get
   $$ [ ce{2C + O2->[1373 K] 2CO} ] times 5 $$ and
   $$ [ ce{5CO + I2O5-> I2 + 5CO2}] times 2 $$
   $$ ce{10C + 5O2 + 2I2O5 -> 2I2 + 10CO2} $$
5. 10 moles (440 g) of of (ce{CO2}) is produced from 5 moles (160 g) of (ce{O2}) obtained from heating of organic compound. (Note: Oxygen from (ce{I2O5}) is not considered here.) It means, 1 mole (44 g) of (ce{CO2}) contains (1 over 2) mole (16 g) of (ce{O2}).
6. If (m_1) g of (ce{CO2}) is produced, then amount of oxygen (sourced from organic compound) is given by $$ frac{text {mass of oxygen (sourced from organic compound) in one mole of carbon dioxide i.e. 16} }{text{molar mass of carbon dioxide i.e. 44 }} times m_1 $$
7. Mass % of Oxygen = ({m_2 over m} times 100)%

Indirect Method

1. Calculate the percentage composition of other elements
2. Subtract the sum of the percentages of all other elements from 100. It is the oxygen %.