-Sugar is dissolved in hot water at 45oC until the mole fraction of water decrea

Question

-Sugar is dissolved in hot water at 45oC until the mole fraction of water decreases to 0.839. The vapor pressure of pure water at this temperature is 72 torr. What is the vapor pressure (in torr) of water for this solution?

-A 0.800 g sample of a nonvolatile yellow crystalline solid is dissolved in 23.0 g of benzene, producing a solution that freezes at 5.10oC. Find the molar mass of the yellow solid in g/mol.

-A 3.40 g sample of a nonvolatile nonelectrolyte blue crystalline solid is dissolved in 39.2 g of acetic acid, producing a solution that bolis at 119.9oC. Find the molar mass of the blue solid in g/mol.

-What is the osmotic pressure in atm of a 0.275 M solution of NaCl at 0oC?

Explanation / Answer

1). 0.839*72 = 60.41

2).

kf of benzene = 4.90
freezing point benzene = 5.5 °C

delta T = 5.5 - 5.1 = 0.4 °C

0.4 = 4.9 x m

m = 0.0816
0.0816 = moles solute / 0.023 Kg

moles solute = 0.0018768

Molar mass = 0.800 g / 0.0018768 mol= 426.26 g/mol

3)

Kb of acetic acid = 3.07
boiling point acetic acid = 118.1 °C

delta T = 119.9 - 118.1 = 1.8 °C

1.8= 3.07 x m

m = 0.586

0.586 = moles solute / 0.0392 Kg

moles solute = 0.02297

molar mass =3.4 g / 0.02297 =148.02 g/mol

4)

The osmotic pressure of a dilute solution can be calculated using the formula:
= iMRT
(i van't Hoff factor of solute, molality of solute, T absolute temperature)

Take care of the units. The unit of volume used for M must match the unit of volume in R. In order to do this it is the best
to use R= 0.08206atmL/molK = 0.08206 atm/MK

NaCl dissociates completely into 2 ions (one Na+ and 1Cl), hence i =2

Therefore:
= 2 0.275mol/m³ 0.08206atmL/molK 273K
=12.32atm

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