How do you determine Sulphur , Phosphorous and Oxygen quantitatively in an organic compound ?

Explanation

Estimation of Oxygen : A known weight of organic compound is decomposed by heating in a stream of nitrogen gas . The mixture of gaseous products containing oxygen is passed over red hot cake to convert all oxygen in those oxides to CO . Then the mixture is passed through hot iodine pentoxide to convert CO into `CO_(2)` and iodine liberates .
Compound `overset("heat")(to)O_(2) +` other gaseous products
`2 C + O_(2) overset(1373 K)(to) 2 CO , I_(2) O_(5) + 5 CO to I_(2) + 5 CO_(2)` (red hot )
Calculation :
Mass of organic compound = a g
Mass of carbon oxide = b g
44 g of `CO_(2)` contains 32 g of oxygen .
`therefore` b g of `CO_(2)` contains ................. ?
`= (b xx 32)/(44)` g of oxygen
a g of organic compound contains `(b xx 32)/(44)` g of `O_(2)` .
`therefore ` 100 g of organic compound contains ......... ?
`(100 xx b xx 32)/(a xx 44)` of oxygen (% of oxygen )