Chemistry
posted by Trixie on .
I am confused about why this is the answer to a practice problem I was given. (answer is c). Could someone explain why/why not for each answer?
2) Solution A is made from 1 L of water and 0.1 moles of Na(CH3COO), and Solution B is made
from 1 L of water and 0.1 moles of Na2(SO4). Assuming both salts completely dissolve and
behave ideally, which of the following is a TRUE statement?
a) Solution A has a higher osmotic pressure than Solution B
b) Solution A has a lower vapor pressure than Solution B
c) Solution A has a higher freezing point than Solution B
d) Solution A has a higher boiling point than Solution B
e) None of the above.

It all has to do with the number of particles in solution.
Solution A is sodium acetate and will have two particles in solutio for every mole of the salt.
Solution B is sodium sulfate and will have 3 particles in solution for every mole of the salt. Can you take it from here or do you want me to draw you a picture? c is the only true statement. 
I see how there are the numbers of particles in solution, I am a little confused on how that relates to these properties. My text is pretty brief over this section.

a) Solution A has a higher osmotic pressure than Solution B
pi = MRT. Effective molarity A is 0.2, effective molarity B is 0.3 so A will give a smaller osmotic pressure than B.
b) Solution A has a lower vapor pressure than Solution B
Psolution = Xsolvent*Pnormal solvent. Since there are 0.2 mol for A and 0.3 mol for B, that means the water is a SMALLER mole fraction for B which means B will have a lower vp.
c) Solution A has a higher freezing point than Solution B
delta T = i*Kf*molality.
i is the van't Hoff factor which is the number of particles. i is smaller for A which means delta T is smaller than for B. A smaller value for A means the lowering of T is less and freezing point is higher.True statement
d) Solution A has a higher boiling point than Solution B.
delta T = i*Kb*m
i is larger for B so delta T will be larger for B and the boiling point will be larger for B and that makes this statement false.
e) None of the above.
Since (c) is the only correct one, (e) can't be true.