Question

a 39.8 degree celcius aqueous solution has a vapor pressure of 47 torr .the solution contains 63.4 g of the non volatile ,non dissociating liquid glycerin and 73.7 g of water.find the VP of water at 39.8 degree celcius?

Answer 1

Glycerine is: C3H8O3

Molar mass of H2O,

MM = 2*MM(H) + 1*MM(O)

= 2*1.008 + 1*16.0

= 18.016 g/mol

Molar mass of C3H8O3,

MM = 3*MM(C) + 8*MM(H) + 3*MM(O)

= 3*12.01 + 8*1.008 + 3*16.0

= 92.094 g/mol

n(H2O) = mass/molar mass

= 73.7/18.016

= 4.091

n(C3H8O3) = mass/molar mass

= 63.4/92.094

= 0.6884

n(H2O),n1 = 4.091 mol

n(C3H8O3),n2 = 0.6884 mol

Total number of mol = n1+n2

= 4.091 + 0.6884

= 4.779 mol

use:

Mole fraction of H2O,

X(H2O) = n1/total mol

= 4.091/4.779

= 0.856

According to Raoult’s law:

P = Po*X(solvent)

p = 47.0*0.856

p = 40.21 torr

Answer: 40. torr