The vapor pressure of pure water is 42.2 Torr at 35°C and is 55.4 Torr at 40°C.

Question

The vapor pressure of pure water is 42.2 Torr at 35°C and is 55.4 Torr at 40°C. What must the molality of a non-electrolyte solute be to produce a solution whose vapor pressure at 40°C is 42.2 Torr?

Select one:

a. 17.36 m

b. 3.22 m

c. 14.18 m

d. 0.556 m

A 15.6-gram sample of magnesium chloride is dissolved in 0.500 kg of water. Determine the colligative molality of the magnesium chloride.

Select one:

a. 0.328 m

b. 0.983 m

c. 0.449 m

d. 1.56 m

Vitamin B12 is a water-soluble vitamin that is a non-electrolyte. Determine the molar mass of the vitamin if a 25.0 mL aqueous solution that contains 6.43 grams of the vitamin produces an osmotic pressure of 4.702 bar at 25°C. [R = 0.083145 L·bar·mol–1·K–1]

Select one:

a. 1360 g·mol–1

b. 121 g·mol–1

c. 556 g·mol–1

d. 11300 g·mol–1

Explanation / Answer

Q1.

The vapor pressure of pure water is 42.2 Torr at 35°C and is 55.4 Torr at 40°C. What must the molality of a non-electrolyte solute be to produce a solution whose vapor pressure at 40°C is 42.2 Torr?

P° = 42.4 torr @ 35°C

P° = 55.4 torr @ 40°C

find molality for a 42.2 torr

Identify the Vapor pressure depression formula

dP = i*(xsolute) * P°solvent

where:

i = van hoff coefficient for mix...

xsolute = molar fraction (NOT molality!)

P°solvent = vapor pressure of solvent at that Temperature

from the data:

P°solvent @ T = 40... P°solvent = 55.4 torr

i = 1 assuming it is not dissociating

Calculate the depression point difference:

dP = P2-P1 = 55.4-42.2 = 13.2 torr

then, solve for xsolute, which is the mole fractino of solute

dP = i*(xsolute)P°solvent

13.2 = 1*(xsolute)55.4

xsolute = 13.2/55.4

xsolute = 0.2382

then...

molality = mol of solute / kg of solvent

Assume a basis of 100 mol so:

0.2382*100 = 23.82 mol are solute

(1-0.2382)*100 = 76.18 mol are solvent

this is water so

MW of Water = 18 g/mol

mass of solvent = mol*MW = 76.18 * 18 = 1371.24 g of solvent

mass of solvent (in kg) = 1371.24/1000 = 1.3712 kg of water

calculate molality

molality = mol of solute / kg solvent = 23.82 /1.3712 = 17.37164 molal

molality of mix in order to have such P° = 17.37164 molal

nearerst answer is A.

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