Question

1-Sugar is dissolved in hot water at 45oC until the mole fraction of water decreases to 0.929. The vapor pressure of pure water at this temperature is 72 torr. What is the vapor pressure (in torr) of water for this solution?

2-54.6 g of the nonvolatile solute urea (M.W. = 60.06 g/mol) is dissolved in 316 g of water at 60oC. The vapor pressure of pure water at this temperature is 149 torr. What is the equilibrium vapor pressure (in torr) of water for this solution?

Answer 1

1)

P = Po*X(water)

P = 72 torr * 0.929

P = 67 torr

Answer: 67 torr

2)

Molar mass of H2O = 2*MM(H) + 1*MM(O)

= 2*1.008 + 1*16.0

= 18.016 g/mol

Molar mass of (NH2)2CO = 60.06 g/mol

n(H2O) = mass/molar mass

= 316.0/18.016

= 17.54

n((NH2)2CO) = mass/molar mass

= 54.6/60.06

= 0.9091

n(H2O),n1 = 17.54 mol

n((NH2)2CO),n2 = 0.9091 mol

Total number of mol = n1+n2

= 17.54 + 0.9091

= 18.449 mol

we have below equation to be used:

Mole fraction of each components are

X(H2O) = n1/total mol

= 17.54/18.449

= 0.9507

According to Raoult’s law:

P = Po*X(solvent)

p = 149.0*0.9507

p = 142 torr

Answer: 142 torr