What would be the normal boiling point of a 1.0 m solution of calcium chloride (

Question

What would be the normal boiling point of a 1.0 m solution of calcium chloride (CaCla) in water (Kb= 0.51 °Cm")? (A) 101.53 C (C) 99.49 °C . (B) 100.51 °C (D) 98.47 °C 10. Which aqueous solution has the lowest freezing point? (A) 1.0 m glucose (C&H1206;) (B) 1.0 m NH4CI (C) 1.0 m K2S (D) All the above solutions have the same freezing point At a given temperature the vapor pressure of ethanol is 44 torr and the vapor pressure of methanol is 94 torr. For an equimolar mixture, the expected vapor pressure of the mixture is 11. (A) 50 tonr (C) 94 torr (B) 69 torr (D) 138 torr

Explanation / Answer

Q9

Apply Colligative properties

This is a typical example of colligative properties.

Recall that a solute ( non volatile ) can make a depression/increase in the freezing/boiling point via:

dTf = -Kf*molality * i

dTb = Kb*molality * i

where:

Kf = freezing point constant for the SOLVENT; Kb = boiling point constant for the SOLVENT;

molality = moles of SOLUTE / kg of SOLVENT

i = vant hoff coefficient, typically the total ion/molecular concentration.

At the end:

Tf mix = Tf solvent - dTf

Tb mix = Tb solvent - dTb

Tb = 100 + (3*1*0.51) = 101.53°C

choose A

Q10

lowest freezing point --> must be the species with most ions in solution, so "i" depresses more

then

K2S = 3 ions, choose this one

Q11

if equimolar:

Ptotal = x1*P1 + x2*P2 = 0.5*44 +0.5*94 = 69

choose B

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