The vapour pressure of pure benzene at a certain temperature is `0.850` bar. A non-volatile, non-electrolyte solid weighting `0.5 g` when added to `39.0 g` of benzene (molar mass `78 g mol^(-1)`). The vapour pressure of the solution then is `0.845` bar. What is the molar mass of the solid substance?

Explanation

To find the molar mass of the solid substance, we can use the formula derived from Raoult's Law for the lowering of vapor pressure:

\[ \frac{P^0 - P_s}{P^0} = \frac{W_2 \cdot M_1}{W_1 \cdot M_2} \]

where:
- \( P^0 \) is the vapor pressure of pure benzene.
- \( P_s \) is the vapor pressure of the solution.
- \( W_2 \) is the mass of the solute.
...