Another way that freezing point depression can be useful is to experimentally de

Question

Another way that freezing point depression can be useful is to experimentally determine the molar mass of an unknown solute in a solution. Use the data in the table to answer the questions below and identify the unknown solute. (a) What mass of solute was used? g (b) What is the freezing point depression DeltaT_f of the solution? degree C (c) What is the colligative molality m_c of the solution? m (d) Assuming a van't Hoff factor i = 1, how many moles of solute are present in solution? mol (e) What is the molar mass of the solute? g/mol

Explanation / Answer

a) mass of solute = mass of solution - mass of solvent

= 10.734 - 10.265 = 0.469 g

b) Tf = Tf(solvent)Tf(solution)

Tf = 25.4 - 21.9 = 3.5 oC

c) molality = Tf / kf

m = 3.5 oC / 9.27 oC / m = 3.77 m

d) moles of solute = 10.265 g * 0.38 mol / 1000 g = 0.0039 mol

e) M.wt = 0.469 g / 0.0039 mol = 120.26 g/mol

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