Question

1. Would it be advisable to determine the freezing point of pure p-dichlorobenzene with one thermometer and the freezing point of the solution with a different one? Explain.
2. If 0.150 mole of a nonvolatile non-electrolyte solute is present in 1200g of water, what are the ideal melting and boiling points of the solution?
3. If the solute above is K2SO4 instead of a covalent solute, what are the ideal (assuming no attraction between oppositely charged ions) melting and boiling points?
4. What weight of ethylene glycol, C2H6O2, must be added to a liter of water to yield a solution that freezes at -15°C?

Answer 1

1. Absolutely not a good idea to use different thermometers. You want the DIFFERENCE in temperature. If either thermometer reads only slightly different than the other, then that error will be the final calculation.

2. m = mols/kg solvent = 0.150/1.2 = ?
Then delta T = Kf*m
Solve for delta T.
For the ideal freezing point subtract delta T from 0 C.

For the ideal b.p., delta = Kb*m add delta T to 100 C.

3. Change delta T = k*m to
delta T = i*Kf*m where i = 3.

4. delta T = Kf*m
You know delta T and Kf for water, solve for m
Then m = mols/kg solvent.
You know m and kg solvent = 1 kg, solve for mols ethylene clycol.
Then mol x molar mass = grams eth glycol