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Question

Your review will be graded on showing all work, all starting equations, good problem solving (minimizing steps and interim answers), correct sig figs, and units.

1) Mathematically determine which will lower the freezing point of water more: 3.0 m NaCl or 2.5 m K2SO4?

2) Mathematically determine which will lower the freezing point of water more: 125.0 grams of KCl or 125.0 grams of Na2SO4? Assume that the amount of ice that is involved for each salt is the same.

mm KCl = 74.55 grams/mole
mm Na2SO4 = 142.04 grams/mole

3) An aqueous solution of an organic (molecular) alcohol has an osmotic pressure of 0.3521 atm. This solution was made by taking 105.32 g of alcohol and dissolving it in enough water to form a 650.0 mL solution at 25.0°C. What is the molar mass of this sample of this organic alcohol?

R = 0.08206 L•atm/mol•K

Explanation / Answer

Ans 1

Freezing point depression

Tf - Ts = i x Kf x m

Freezing point depression constant Kf = 1.86°C/m

For NaCl, i = 2

Molality m = 3

Tf - Ts = 6*kf = 11.16

Tf = 11.16 + Ts ........ Eq1

For K2SO4, i = 3

m = 2.5

Tf - Ts = 7.5 * kf = 13.95

Tf = 13.95 + Ts ...... Eq2

On comparing eq1 and eq2

Tf will be lower for eq 1

3 m NaCl will lower the freezing point of water more

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