Question

At 29.6 c, pure water has a vapor pressure of 31.1 torr. A solution is prepared by adding 86.8 g of "y", a non-volatile non-electrolyte to 350. G of water. The vapor pressure of the resulting solution is 28.6 torr. Calculate the molar mass of y.

Answer 1

Solution:

1) Use Raoult's Law to determine mole fraction of the solvent:

Psolution = (χsolvent) (P°solvent)

28.6 torr = (χsolvent) (31.1 torr)

χsolvent = 28.6 torr / 31.1 torr

χsolvent = 0.91961415

2) Use the mole fraction and the moles of water to determine the moles of Y:

350. g / 18.015 g/mol = 19.428254 mol

0.91961415 = 19.428254 / (19.428254 + x) <--- x equals the moles of Y dissolved

(0.91961415) (19.428254 + x) = 19.428254

17.8665 + 0.91961415x = 19.428254

0.91961415x = 1.561754

x = 1.69827 mol

3) Calculate molar mass of Y:

86.8 g / 1.69827 mol = 51.1 g/mol (to three sig figs)