Question

5.8 g of a non-volatile solute was dissolved in 100g of carbon disulphide $\left(C{S}_2\right)$. The vapour pressure of the solution of the solution was found to be 190 mm of Hg. Calculate molar mass of solute given the vapour pressure of $\left(C{S}_2\right)$ is 195 mm of Hg.
[molar mass of $\left(C{S}_2\right)$ = 76 g mol${}^{-1}$]

Answer 1

We have,

$\frac{P_A^0-P_S}{P_A^0}=x_A$

let molar mass of solute = M

so, moles of solute = $\frac{5.8}{M}=x$

and moles of solvent= $\frac{100}{76}=1.31$

we get,

$\frac{195-190}{195}=\frac{x}{x+1.31}$

$x=0.0344moles$

we have,

$\frac{5.8}{M}=0.034$

thus molar mass of solute = 170.6$