Question

The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non-volatile, non-electrolyte solid weighing 0.5 g is added 39.0 g of benzene (molar mass 78 g/mol) the vapour pressure of the solution then is 0.845 bar. what is the molar mass of the solid substance.​

Answer 1

Answer :

• Molar mass of the solid substance = $\bold{170\:g\:mol^{-1}}$

Step-by-step explanation :

Given that,

• Vapour pressure of pure benzene($\bold{p^{\circ}}$) = 0.850 bar.
• Vapors pressure of the solution($\bold{p_s}$) = 0.845 bar.
• Molar mass of benzene($\bold{M_1}$) = 78 g/mol.
• Weight of solid = 0.5 g.
• Weight of benzene = 39.0 g.

$\hrulefill$

Substituting the given values in the formula,

$\bold{\dfrac{p^{\circ}-p_s}{p^{\circ}}=\dfrac{n_2}{n_1}=\dfrac{w_2/M_2}{w_1/M_1}=\dfrac{w_2M_1}{w_1M_2}}$

We get,

$\bold{\dfrac{0.850\:bar-0.845\:bar}{0.850\:bar}=\dfrac{0.5\:g\times{}78\:g\:mol^{-1}}{39.0\:g\times{}M_2}}$

Then, $\bold{M_2=\dfrac{0.5\times{}78}{39.0}\times{}\dfrac{0.850}{0.005}g\:mol^{-1}}$

$\implies\bold{M_2=170\:g\:mol^{-1}}$