AP Chemistry states of matter help!?

Question

a 0.562 g sample of an unkown substance was dissolved in 17.4 g benzene. the freezing point of the solution was 4.075 degrees celcius. the freezing point of pure benzene is 5.455 degrees celcius. for benzene, Kf=5.065 degrees celcius/m and Kb=2.61 degrees celcius/m. assume that the solute is a non-electrolyte.

a. what is the molality of the solution?
b. what is the molar mass of the unkown?
c. if the boiling temperature of pure benzene is 80.2 degrees celcius, what is the boiling temperature of the solution?
d. what is the van't Hoff factor? explain whether one was neccessary in these calculations

Answer 1

The colligative property formula is:

delta T = m Kf, where m is molals

You have delta T below the normal freezing point and you have Kf.

So solve for molals.

molals = moles solute / kg. of solvent. You have 17.4 kg of benzene solvent. Now you can solve for moles of solute.

You have 0.562 g of solute, now you can solve for the molecular mass.

You have the Kb and the molals. Now you can solve for the delta T in higher boiling point aboe 80.2 degrees.

Don't know about van't hoff. Look it up

Answer 2

So you know that ∆T = i x Kf x molality --
∆T is the change in the freezing point
i is the number of individual particles (usually ions) formed when one molecule dissolves (the van't Hoff factor)
Kf is the freezing point depression constant
molality is the moles of dissolved solute per kilogram of solvent
The unknown is a non-electrolyte, which means that it does not turn into ions when dissolved, so only one "particle" for each dissolved molecule
∆T = 5.455°C - 4.075°C
i = one
Kf = 5.065°Ckg/mol

a) molality = ∆T/Kf
b) since moles solute = grams solute/molar mass solute, molality x kilograms solvent = grams solute/molar mass solute.
c) ∆T (change in boiling point) = Kb x molality
use the molality you just calculated to find the ∆T, then ADD this to the boiling point.
d) (see i above)since this is a non-electolyte (no ions), the van't Hoff factor is not needed.

Answer 3

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