The mass of Non-volatile solute of molar mass 40 g/mole. Which on dissolution in 57 g octane (${\mathrm{C}}_8{\mathrm{H}}_{18}$ ) to reduce the vapour pressure of octane by 20% is ________ .

Mass of solute$={\mathrm{W}}_1=?$

Mass of solute$={\mathrm{M}}_1=40$

Mass of solvent$={\mathrm{M}}_2=57$

Mass of solvent$={\mathrm{M}}_2=114$

Octane=${\mathrm{C}}_8{\mathrm{H}}_{18}$=96+18=114

$\frac{{\mathrm{P}}^0-\mathrm{P}}{{\mathrm{P}}^0}=\frac{20}{100}$

$\frac{{\mathrm{P}}^0-\mathrm{P}}{{\mathrm{P}}^0}=\frac{n_1}{n_2}$(For dilute solutions)

$\frac{20}{100}=\frac{W_1}{M_1}\times \frac{114}{57}$

${\mathrm{W}}_1=\frac{20\times 40\times 57}{100\times 114}=4$