Question

1. Find the molality of the solution prepared by dissolving 0.238g toluene, C7H8, in 15.8g cyclohexane

2. A pure sample of the solvent phenol has a freezing point of 40.85 degrees C. A 0.414 molal solution of isopropyl alcohol was observed to have a freezing point of 38.02 degrees C
(a) find the freezing point depression of the solution
(b) calculate the freezing point depression constant of phenol

3. A 0.996 g sample of an unknown was dissolved in 10.1 g benzene. If the freezing point depression of the solution was 4.19 degrees C, find the molar mass of the known.

Answer 1

1. formula for Molality = n(solute)in mol/(m(solvent)) in (Kg)

so: first convert mass of C7H8 into moles, then divide that by the mass of cyclohexane, which in this case is 15.8 g, then multiply by 1000 to convert to Kg. Report the answer to three significant figures.

Ans: moles of C7H8 = 0.238g C7H8 * (1 mol C7H8/(92g C7H8)) = 0.002586 mol C7H8

Molality of Solution = 0.002586 mol C7H8 * (1/15.8g cyclohexane)* (1000g/1Kg) = 0.164 mol/Kg solution.

Answer 2

ok i got that. any ideas on #3?

Answer 3

Look up the freezing point constant for bernzene (Kf). Delta T is given in the problem as 4.19C.
Then delta T = Kf*m
Substitute delta T and Kf and solve for m

m = mols solute/kg solvent. You know m and kg solvent, solve for mols solute.

mols solute = grams/molar mass. You know mols solute and grams, solve for molar mass.

Answer 4

This doesn't answer number 2 and I'm really confused on how to do it. Could anyone possible answer # 2. I'm not asking for C only A and B

Answer 5

TION 1:
Consider the following equilibrium reaction at 700(C.
2H2 (g) + S2 (g) 2H2S (g)
Analysis of the equilibrium mixture shows that there are 2.5 mole of H2 (g), 1.35 x 10-5 moles of S2 (g) and 8.7 moles of H2S (g) present in a container of volume 12 liters:
Calculate the equilibrium constant Kc for the reaction
Calculate the equilibrium constant Kp