Question

If $M_{\text{normal }}$ is the normal molecular mass and $\alpha$ is the degree of ionization of $K_3\left[Fe(CN{)}_6\right],$ then the abnormal molecular mass of complex is:

• A. $M_{\text{normal }}(1+3\alpha {)}^{-1}$
• B. $M_{\text{normal }}(1+2\alpha {)}^{-1}$
• C. $M_{\text{normal }}(1+\alpha {)}^{-1}$
• D. None of these

Answer 1

Answer: (A)

$K_3\left[Fe(CN{)}_6\right]$ ionises as

$K_3\left[Fe(CN{)}_6\right]\to 3K^{+}+{\left[Fe(CN{)}_6\right]}^3$

For dissociation,

$\alpha =\frac{i-1}{n-1}$

$\therefore (3\alpha +1)=i$

Now,

$i=\frac{\text{ Normal molar mass of solute }}{\text{ Abnormal molar mass of solute }}$

$1+3\alpha =\frac{\text{ Normal molar mass of solute }}{\text{ Abnormal molar mass of solute }}$

Abnormal molar mass of solute

$=M_{\text{normal }}(1+3\alpha {)}^{-1}$