What mass, in grams, of NaCl needs to be added to 1.6 kg of water in order to create a solution with a freezing point of -5.6 °C? The freezing point depression constant of water is 1.86 ºC/m

Answer 1

∆T = imK

∆T = change in freezing point = 5.6º

i = van't Hoff constant = 2 for NaCl because it dissociates into TWO particles, Na and Cl

m = molality of NaCl = moles/kg solvent =?

K = freezing point constant =1.86º/m

m = ∆T/(i)(K)

m = 5.6/(2)1.86)

m = 1.51 mols/kg

Since we have 1.6 kg, that means we have....

1.51 mol/Kg x 1.6 kg = 2.42 moles of NaCl

molar mass NaCl = ~ 58.4 g/mol

mass (in grams) of NaCl = 2.42 mol NaCl x 58.4 g/mol = 141 g NaCl