Question

What will be the abnormal molecular mass of ${\text{K}}_3\left[\text{Fe}{\left(\text{CN}\right)}_6\right]$. If in a aqueous solution it is 40% dissociated? (Given, normal molecular weight of ${\text{K}}_3\left[\text{Fe}{\left(\text{CN}\right)}_6\right]$ is 329)

• A. $149.54$
• B. $723.8$
• C. $82.25$
• D. $329.0$

Answer 1

The correct option is A $149.54$

$\underset{1-0.4}{\underset{1}{{\text{K}}_3\left[\text{Fe}{\left(\text{CN}\right)}_6\right]}}\to \underset{3\times 0.4}{\underset{}{3{\text{K}}^{+}}}+\underset{0.4}{{\underset{}{\left[\text{Fe}{\left(\text{CN}\right)}_6\right]}}^{3-}}$
Van't Hoff factor, $\text{i}=1-0.4+\left(3\times 0.4\right)+0.4$
= 1 + 1.2
= 2.2
$\text{i}=\frac{\text{Normal molar mass}}{\text{Abnormal molar mass}}$
Abnormal molar mass = $\frac{\text{Normal molar mass}}{\text{i}}$
$=\frac{329}{2.2}$
= 149.54
Thus, correct option is (A).