Question

Consider the following statement and select correct option :
(l) $K_{sp}$ of $Fe(OH{)}_3$ in aqueous solution is 3.8 x ${10}^{38}$ at 298 K. The concentration of $F{e}^{3+}$ will increase when $\left[H^{+}\right]$ ion concentration decreases.
(ll) In a mixture of $N{H}_{4}Cl$ and $N{H}_{4}OH$ in water, a further amount of $N{H}_{4}Cl$ is added. The pH of the mixture will decreases.
(lll) An aqueous solution of each of the following salts $(N{H}_{4}l,HCOOK)$ will be basic, acidic respectively.

• A. only l is correct
• B. only ll is correct
• C. only lll is correct
• D. ll and lll are correct

Answer 1

The correct option is A only l is correct

(I) $K_{sp}=\left[F{e}^{3+}\right][O{H}^{-}{]}^3$,Here to maintain the $K_{sp}$ at its value, if $\left[F{e}^{3+}\right]$ increases then $\left[O{H}^{-}\right]$ ion will decrease.

(II) $pH$ for basic buffer= $14-p{K}_b+\log \frac{\left[N{H}_{4}OH\right]}{\left[N{H}_{4}Cl\right]}$

If $N{H}_{4}Cl$ is added, then $\left[N{H}_{4}Cl\right]$ will increase, due to which the logarithmic term may become negative and $pH$ will decrease.

(III) $N{H}_{4}I$ is salt of weak base & strong acid, hence its solution will be acidic. Similarly, $HCOOK$ is a salt of strong base & weak acid, hence its solution will be basic.