what mass of NaCl is required to lower the melting point of 5.5 kg of ice to -5.0 ºC?
In ice-cream making, the temperature of the ingredients is kept below 0 ºC using an ice-salt bath. Assuming that NaCl dissolves completely and forms an ideal solution, what mass of NaCl is required to lower the melting point of 5.5 kg of ice to -5.0 ºC? [Kf (H2O) = 1.86 ºC/m]
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NaCl is a strong electrolyte : NaCl = na+ + Cl- for which i = Vant'Hoff factor = 2
delta T = i x m x kf
5.0 = 2 x m x 1.86
m = 1.3 = moles solute / mass water = moles NaCl / 5.5
moles NaCl = 1.3 x 5.5=7.15
mass NaCl = 7.15 mol x 58.44 g/mol=417.9 g
60 grams