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

$K _{3}\left[ Fe ( CN )_{6}\right] \rightarrow 3 K ^{+}+\left[ Fe ( CN )_{6}\right]^{3}$

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

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