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 non-volatile, non-electrolyte solid added to benzene, we can use Raoult's law, which relates the vapor pressure of the solution to the vapor pressure of the pure solvent and the mole fraction of the solvent in the solution.

### Step-by-Step Solution:

1. **Identify Given Values:**
- Vapor pressure of pure benzene (P₀) = 0.850 bar
- Vapor pressure of the solution (Pₛ) = 0.845 bar
- Mass of the solute (solid) = 0.5 g
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