Question

Calculate the molality of KI if the density of 20% (mass/mass) aqueous KI is 1.202 gmL^-1. Report your final answer by rounding off the molality correct upto one place of decimal and then multiplying it by 10.

Answer 1

Weight of KI in 100g of water = 20 g

Weight of water in the solution = 100 - 20 = 80 g= 0.08 kg

Molar mass of KI = 39 + 127 = 166 g mol^-1

$\text{Molality of the solution (m)} = \frac{\text{Number of moles of KI}}{\text{Mass of water in kg}}$

$=\frac{(20 \mathrm{~g}) /\left(166 \mathrm{~g} \mathrm{~mol}^{-1}\right)}{(0 \cdot 08 \mathrm{~kg})}$

= 1.506 mol kg^-1 = 1.506 m

By rounding off to one place, we get the molality as 1.5 and by further multiplying it with 10, we get "15" as the final answer.