In icre-cream making, the ingredients are kept below 0.0 degree Cin an icre-salt
ID: 76734 • Letter: I
Question
In icre-cream making, the ingredients are kept below 0.0 degree Cin an icre-salt bath.a) assuming that NaCl dissolves completely and forms an idealsol'n, what mass of it is needed to lower the melting point of 5.5kg of ice to -5.0 degree C
b) Given the same assumptions as in part a) what mass ofCaCl2 is needed?
Explanation / Answer
T = van't Hoff*Kf*molality Kf = 1.86 C/m T = 5 C van't Hoff = 2 (the number of ions a species dissolves into; NaCl=> Na+ + Cl- molality = T/(Kf*van't hoff) = 5 C/(1.86 C/m *2) = 1.344molal = 1.344 moles NaCl/ kg 5.5 kg ice *(1.344 mol NaCl/kg) *(58.44 g /mol NaCl) = 431.98848 gNaCl ~ 4.3 x 102 g b) van't HOff = 3 in this case : CaCl2 => Ca2+ + 2Cl- molality = T/(Kf*van't hoff) = 5 C/(1.86 C/m*3) = 0.896 molal 5.5 kg ice *(0.896 mol NaCl/kg) *(58.44 g /mol NaCl) = 287.99 gNaCl ~ 2.9 x 102 g