Answer to Question #185632 in General Chemistry for Joshua

Question

A solution containg 4.50 g of a non electrolyte (i=1) dissolved in 125g of water freezes at -0.372°C. Calculate the molar mass of the solute.

Step 1: Firsy compute the molarity of the freezing point equation

Step 2: Then, from the definition of molality, compute the number of moles solute, n(solute), in the sample.

Step 3: Solve for the molar mass

Answer 1

∆T = K_F·Cm

where K_F is the molal freezing point depression constant (for water K_F = 1.86);

Cm is the molality of the solute.

Cm = ∆T / K_F = 0.372 / 1.86 = 0.2 m

Cm = n / m

n = Cm x m = 0.2 x 0.125 = 0.025 mol

n = m / M

M_solute= m / n = 4.50 / 0.025 = 180 g/mol