Question

What is the vapor pressure at  75°C  of an aqueous solution prepared by the addition of  64.3  g of the nonvolatile solute urea, CO(NH2)2CO(NH2)2, to  584  g of water? The vapor pressure of pure water at  75°C  is  290. mmHg.

Select one:

a. 133 mmHg

b. 28.9 mmHg

c. 257 mmHg

d. 261 mmHg

e. 281 mmHg

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 C2O2N4H8,

MM = 2*MM(C) + 2*MM(O) + 4*MM(N) + 8*MM(H)

= 2*12.01 + 2*16.0 + 4*14.01 + 8*1.008

= 120.124 g/mol

n(H2O) = mass/molar mass

= 584.0/18.016

= 32.4156

n(C2O2N4H8) = mass/molar mass

= 64.3/120.124

= 0.5353

n(H2O),n1 = 32.4156 mol

n(C2O2N4H8),n2 = 0.5353 mol

Total number of mol = n1+n2

= 32.4156 + 0.5353

= 32.9509 mol

Mole fraction of each components are

X(H2O) = n1/total mol

= 32.4156/32.9509

= 0.983

According to Raoult’s law:

P = Po*X(solvent)

p = 290.0*0.983

p = 281 mmHg

Answer: e