Complete combustion of 0.858 g of compound X gives 2.64g of CO₂ and 1.26g of H₂O. The lowest molecular mass X can have:

Moles of $\mathrm{C}={\mathrm{n}}_{{\mathrm{CO}}_2}=\frac{2.64}{44}=0.06$

$\Rightarrow \text{ Mass of }\mathrm{C}=0.72$

Mole of $\mathrm{H}=2\times \text{ Moles of }{\mathrm{H}}_2\mathrm{O}=2\times \frac{1.26}{18}=0.14\Rightarrow \text{ mass of }\mathrm{H}=0.14$

Compound does not contain oxygen

So $\mathrm{EF}:{\mathrm{C}}_{0.06}{\mathrm{H}}_{0.14}={\mathrm{C}}_3{\mathrm{H}}_7$

$\therefore \text{ Lowest }\mathrm{Mw}=43$