Question

333.7g of NaCl completely dissolves (producing Na + and Cl - ions) in 1.00 kg of water at 25.0 degrees celsius. The vapore pressure of pure water at this temperature is 23.8 torr. Determine the vapor pressure of the solution.

A) 18.2 torr

B) 23.8 torr

C) 19.7 torr

D) 17.5 torr

E) 21.6 torr

I know the answer is C, but why? (Show work and explain please).

Answer 1

Molar mass of H2O,

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

= 2*1.008 + 1*16.0

= 18.016 g/mol

Molar mass of NaCl,

MM = 1*MM(Na) + 1*MM(Cl)

= 1*22.99 + 1*35.45

= 58.44 g/mol

n(H2O) = mass/molar mass

= 1000.0/18.016

= 55.51

n(NaCl) = mass/molar mass

= 333.7/58.44

= 5.71

It breaks into 2 componnet.

So, i = 2

n(H2O),n1 = 55.51 mol

n(ions),n2 = 2*5.71 = 11.42 mol

Total number of mol = n1+n2

= 55.51 + 11.42

= 66.93 mol

use:

Mole fraction of H2O:

X(H2O) = n1/total mol

= 55.51/66.93

= 0.8294

According to Raoult’s law:

P = Po*X(solvent)

p = 23.8*0.8294

p = 19.7 torr

Answer: 19.7 torr