chemistry 102 college

Question

What mass of glycerin (C₃H₈O₃) must be dissolved in 171.0 g water to give a solution with a freezing point of –3.23°C? K_f for water = 1.86 °C∙kg/mol

Determine the molality of C₃H₈O₃.



Answer 1

∆T = imK

∆T = change in freezing point = -3.23

i = van't Hoff factor for glycerin = 1 (non electrolyte)

m = molality of glycerol = mols/kg water = ?

K = freezing constant for water = -1.86º/m

Solving for m, we have...

-3.23 = (1)(m)(-1.82)

m = 1.77 moles glycerol / kg water

We have 171.0 g of water = 0.171 kg water

1.77 mols glycerol / kg x 0.171 kg = 0.303 mols glycerol needed

mass glycerol = 0.303 mols x 92.1 g / mol = 28.0 g glycerol (3 sig. figs.)