Solutions and Colligative Properties - Result Question 31

####2. What would be the molality of $20 %$ (mass/mass) aqueous solution of KI? (Molar mass of KI = $166 g mol^{-1}$ )

(2019 Main, 9 April I)

(a) 1.48

(b) 1.51

(c) 1.35

(d) 1.08

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Solution:

  1. The expression of elevation of boiling point,

$$ \begin{aligned} \Delta T _b & =K _b \times m \times i \ & =k _b \times \frac{w _2 \times 1000}{M _2 \times w _1} \times i \end{aligned} $$

where, $m=$ molality

$i=$ van’t Hoff factor $=1$ (for non-electrolyte/non-associable)

$w _2=$ mass of solute in $g=1 g$ (present in both of the solutions)

$M _2=$ molar mass of solute in $g mol^{-1}$ (same solute in both of the solutions)

$w _1=$ mass of solvent in $g=100 g$ (for both of the solvents $A$ and $B$ )

$K _b=$ ebullioscopic constant

So, the expression becomes,

$$ \begin{aligned} & \Delta T _b \propto K _b \ & \Rightarrow \quad \frac{\Delta T _b(A)}{\Delta T _b(B)}=\frac{K _b(A)}{K _b(B)}=\frac{1}{5} \quad\left[\text { Given } \frac{K _b(A)}{K _b(B)}=\frac{1}{5}\right] \end{aligned} $$



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