Question

What is the vapor pressure at 20 °c of an ideal solution prepared by the addition of 7.38 g of the nonvolatile solute urea, co(nh2)2, to 56.9 g of methanol, ch3oh? the vapor pressure of pure methanol at 20 °c is 89.0 mm hg.

a. 5.75 mm hg

b?

Answer 1

Answer:

83.24 mmHg.

Explanation:

• The vapor pressure of the solution (Psolution) = (Xmethanol)(P°methanol).

where, Psolution is the vapor pressure of the solution,

Xmethanol is the mole fraction of methanol,

P°methanol is the pure vapor pressure of methanol.

• We need to calculate the mole fraction of methanol (Xmethanol).

Xmethanol = (n)methanol/(n) total.

where, n methanol is the no. of moles of methanol.

n total is the total no. of moles of methanol and urea.

• We can calculate the no. of moles of both methanol and urea using the relation: n = mass/molar mass.

n of methanol = mass/molar mass = (56.9 g)/(32.04 g/mol) = 1.776 mol.

n of urea = mass/molar mass = (7.38 g )/(60.06 g/mol) = 0.123 mol.

∴ Xmethanol = (n)methanol/(n) total = (1.776 mol)/(1.776 mol + 0.123 mol) = 0.935.

∴ Psolution = (Xmethanol)(P°methanol) = (0.935)(89.0 mmHg) = 83.24 mmHg.