If isobutane and n-butane are present in a gas, then how much oxygen should be required for complete combustion of 5 kg of this gas

Isobutane and n-Butane have same chemical formula (C₄​H₁₀​) and give same combustion reaction.

2C₄​H₁₀ ​+ 13O₂ ​→ 8CO₂ ​+ 10H₂​O

This means 2 moles of this mixture requires 13 moles of oxygen.

Molecular mass of butane = 58 g/mol

Molecular mass of oxygen = 32 g/mol

Thus, 116 g mixture needs 13 × 32 g=416 g oxygen

1 g mixture needs = $\frac{416}{116}$ ​g oxygen
5 kg  of the mixture  = 5 × 10³ g needs = $\frac{416\times 5\times {10}^3}{116}$​ = 17900 g of oxygen

Therefore, 17.9 kg oxygen is used.