Question

benzene at certain temperature is 640 mm of HG a non-volatile solid weighing 2.1 75 grams is added to 39.0 8 of benzene the vapour pressure of solution is 600 mm of HG what is the molecular weight of solid substance

Answer 1

mass of solid substance = 2.175g

mass of benzene = 39.08g

molar mass of benzene = 78g/mol

so, mole fraction of solute = n/(n + N)

where n is mole of solute and N is mole of solvent (benzene).

N = 39.08/78 = 0.5

from lowering of vapor pressure,

change in vapor pressure/vapor pressure of pure benzene = mole fraction of solute

or , (640 - 600)/640 = n/(n + 0.5)

or 40/640 = n/(n + 0.5)

or, n + 0.5 = 16n

or, n = 0.5/15 = 1/30

or , 2.175/M = 1/30, where M is molecular weight of solid substance.

or, M = 2.175 × 30 = 65.25 g/mol