1. Combustion of Carbon

$C_{\left(s\right)}+O_{2\left(g\right)}\to {CO}_{2\left(g\right)}$

$12kg$ of carbon requires $32kg$ of oxygen for complete combustion

For complete combustion, the fuel's carbon needs oxygen $=$$\frac{32}{12}\times Ckg$

Oxygen is required for the complete combustion of $C$ kg of carbon in the fuel $=$$\frac{8}{3}Ckg$

2. Combustion of Hydrogen

$2H_{2\left(g\right)}+O_2\to 2H_2{O}_{\left(l\right)}$

$4$ $kg$ of hydrogen requires $32$ $kg$ of oxygen for complete combustion

For complete combustion, the fuel's hydrogen needs oxygen$=$$\frac{32}{4}\times Hkg$

Oxygen is required for the complete combustion of $H$ kg of carbon in the fuel$=$$8H kg$

NOTE- If the fuel already contains some oxygen, the oxygen content must be subtracted from the overall mass.

The total mass of oxygen required per kg of fuel$=$$\left(\frac{8}{3}C+8H+S\right)-O_2$ 

Mass percentage of oxygen 

The mass percentage of oxygen in air=$23\%$

$23\%$ of Oxygen is supplied by $100\%$ the mass of air

Therefore, $23kg$ of oxygen $=$ $100kg$ of air required

Hence, $1kg$ of oxygen $=$ $\frac{100}{23}kg$ of air

For the burning of $1kg$ of fuel, a minimum mass of air is needed $=$ $\left(\left(\frac{8}{3}C+8H+S\right)-O_2\right)\times \frac{100}{23}kg$