Question

How much amount of NaCl should be added to
600 g of water (p= 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

Given:

• The mass of solvent, i.e. water (w2) = 600 g
• The value of Kf for water = 2 K.Kg/mol
• Decrease in freezing point of water (ΔTf) = 0.2K
• Molar mass of NaCl (Mm) = 58.5 g/mol

To find:

The amount of NaCl required.

Solution:

• Let the amount of NaCl required be w g.
• Molality (m) = (w*1000)/(Mm*w2) = 1000w/(600*58.5) = 0.0285w
• Since, NaCl is completely soluble in water, the Van't Hoff factor(z) = 2
• ΔTf = z*m*Kf ⇒ 0.2 = 2*0.0285w*2 ⇒ w = 0.2/(4*0.0285) = 1.75 g

Answer:

The amount of NaCl required = 1.75 g