Question

27.Choose the aqueous solution below with the lowest freezing point. These are all solutions of nonvolatile solutes and you should assume ideal van't Hoff factors where applicable. A. 0.075 m NaI B. 0.075 m (NH4)3PO4 C. 0.075 m NaBrO4 D. 0.075 m LiCN E. 0.075 m KNO2

Answer 1

Delta T_f = K_f * m * i where, Delta Tf = the amount the freezing temperature decrease kf = the freezing point depression constant which depends on the solvent (for water, kf = 1.86 degree C/m) m = the molality of the solution i = the number of particles formed when that compound dissolves (for covalent compounds, this number is always 1) Let's calculate depression in freezing point of all solution, solution having largest Delta Tf will have lowest freezing point: for all solutions m = 0.075 m A) Nal: i = 2 because it dissociated into two ions, Na + and I- Delta Tf = 1.86*0.075*2 = 0.279 degree C B) (NH4) 3PO4: i = 4 (3* NH4 + and 1* PO4- ions) Delta Tf = 1.86*0.075*4 = 0.558 degree C C) NaBro4: i = 2 (1*Na + and 1*BrO4- ions) Delta Tf = 1.86*0.75*2 = 0.279 degree C D) LiCN: i = 2 (Li + and -CN) Delta Tf = 1.86*0.075*2 = 0.279 degree C E) KNO2: i = 2, (K + and NO2- ions) Delta Tf = 1.86*0.075*2 = 0.279 degree C Thus largest