Question

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

(a) 58
(b) 180
(c) 170
(d) 145

Answer 1

To find the molecular mass of the solid substance, we use Raoult's law to calculate the mole fraction of benzene in the solution and then find the mole fraction of the solid substance. Finally, we can calculate the molecular mass of the solid substance.

Step-by-step explanation:

To find the molecular mass of the solid substance, we can use Raoult's law to calculate the mole fraction of benzene in the solution:

Mole fraction of benzene = (vapor pressure of benzene in solution) / (vapor pressure of pure benzene)

Plugging in the values, we get:

Mole fraction of benzene = 0.845 bar / 0.850 bar = 0.9941

Next, we can use the mole fraction to find the mole fraction of the solid substance:

Mole fraction of solid substance = 1 - Mole fraction of benzene = 1 - 0.9941 = 0.0059

Finally, we can calculate the molecular mass of the solid substance:

Molecular mass of solid substance = (mass of solid substance) / (number of moles of solid substance)

Plugging in the values, we get:

Molecular mass of solid substance = (0.5 g) / (0.0059 mol) = 85.59 g/mol

Therefore, the molecular mass of the solid substance is approximately 86 g/mol.