Question

Calculate the vapor pressure of a solution containing 28.0 g of glycerin (c3h8o3) in 120 ml of water at 30.0 âc. the vapor pressure of pure water at this temperature is 31.8 torr. assume that glycerin is not volatile and dissolves molecularly (i.e., it is not ionic) and use a density of 1.00 g/ml for the water.

Answer 1

First step is to get the number of moles of glycerin in 28 gm:
From the periodic table:
molar mass of carbon = 12
molar mass of oxygen = 16
molar mass of hydrogen = 1
molar mass of glycerin = 3(12) + 8(1) + 3(16) = 92 gm
number of moles = 28/92 = 0.304 moles

Second step is to get the number of moles of water:
mass = 120 ml x 1 gm/ml = 120 gm
molar mass of water = 2(1) + 16 = 18 gm
number of moles = 120/18 = 6.667 moles

Third step is to get the mole fraction of water:
mole fraction of water =
number of water moles / (number of water moles+number of glycerin moles)
mole fraction of water = (6.667) / (6.667+0.304) = 0.956

Last step is to calculate the pressure:
p = vapor pressure of pure water x mole fraction of water
      31.8 x 0.956 = 30.4008  torr