chemistry

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.

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asked by kales
  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.

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    posted by Scott
  2. ok i got that. any ideas on #3?

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    posted by kales
  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.

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    posted by DrBob222
  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

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    posted by Brytni

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