Question

How much amount of NaCl should be added to 600 g of water (ρ= 1.00 g/mL) to decrease the freezing point of water to – 0.2 °C ? ______. 

(The freezing point depression constant for water = 2K kg mol–1)

Answer 1

ΔT_f = i x m K_f

0.2 = 2 x 2 x (w/58.5)/(600/1000)

w = 1.755gm

Answer 2

ΔT_f = 0.2 = 2 x n/600 x 1000 x 2

n = 0.1 x 0.6/2 = 0.03moles

mass of Nacl = 0.03 x 58.5

= 1.755