Question

Determine the mass of sucrose (C₁₂H₂₂O₁₁) needed to lower the freezing point of 100.0 g of water by 3.00 ^oC. (K_f = 1.86 ^oC kg/mol).

Answer 1

Depression in the freezing point =

,            where Kf = 1.86 ^oC. kg/mol and m= molality of the solution

Given

3 = 1.86 (m)

molality = 1.61 m

But

Molality = moles of the solute/ weight of the solvent(kg)

weight of the solvent = 100 g = 0.100 L

1.61 = moles /0.100

Moles of the solute, sucrose = 0.161 moles

Molar mass of sucrose (C12H22O11) = 342.3 g/mol

Mass of the sucrose =

Mass of sucrose needed = 55.2 g