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Question

The following data were collected for three compounds in aqueous solution. Determine the value of the van’t Hoff factor for each salt. (K_f for water is 1.86°C/m) for the experimentally measured ΔT_f value.

Compound: LiCl

Concentration: 5.00g/kg

Measured ΔT_f: 0.328°C

Compound: HCl

Concentration: 4.00g/kg

Measured ΔT_f: 0.389°C

Compound: NaCl

Concentration: 6.00g/kg

Measured ΔT_f: 0.239°C

Answer 1

Colligative Properties

∆T = imK

∆T = change in temperature

i = van't Hoff factor

m = molality = moles solute / kg solvent

K = 1.86º/m

LiCl molar mass = 42.4 g/mol

moles LiCl = 4.00 g x 1 mol / 42.4 g = 0.0943 moles

kg solvent = 1

m = 0.0943 mol/1 kg = 0.0943 m

∆T = 0.328º

Solving for i, we have...

i = ∆T / (m)(K) = 0.328 / (0.0943)(1.86)

i = 1.87 (theoretical value would be 2)

Do the HCl and NaCl in the same manner as was done for LiCl