Question

Determine the $pH$ of a $0.2M$ solution of pyridine $C_5{H}_{5}N$.

[For pyridine : $K_b=1.5\times {10}^{-9}$]

• A. $pH=4.76$
• B. $pH=8.43$
• C. $pH=9.24$
• D. $pH=11.73$

Answer 1

The correct option is C $pH=9.24$

We know that, by Ostwald's Dilution Law,
$K_b={\alpha }^{2}C$
${\alpha }^2=\frac{K_b}{C}=\frac{1.5\times {10}^{-9}}{0.2}$ $=$ $75\times {10}^{-10}$
$\alpha =8.66\times {10}^{-5}$
$\left[O{H}^{-}\right]$ $=$ $n\times \alpha \times C=1\times 8.66\times {10}^{-5}\times 0.2=1.732\times {10}^{-5}mol/d{m}^3$
where, n=1
$pOH=-lo{g}_{10}\left[O{H}^{-}\right]$ $=$ $-lo{g}_{10}(1.732\times {10}^{-5})=5-0.24=4.76$
$pH=14-pOH=14-4.76=9.24$