MCQ
$0.1$ $mole$ of $CH_3NH_2 (K_b = $ $5 \times 10^{-4} $ $)$ is mixed with $0.08 $ $mole$ of $ HCl $ and diluted to one litre. What will be the $H^+ $ concentration in the solution?
  • A
    $8 × 10^{-2}\, M$
  • $8 × 10^{-11}\, M$
  • C
    $1.6 × 10^{-11} \,M$
  • D
    $8 × 10^{-5} \,M$

Answer

Correct option: B.
$8 × 10^{-11}\, M$
(b) $C{H_3}NH_2 + HCL$  $\xrightarrow{{}}$$C{H_3}NH_3^ + C{l^ - }$

        $0.1$                            $0.08$                    $ 0$

        $0.02$                          $0$                           $0.08$

(Basic buffer solution)

$pOH = pK_b + log$  $\frac{{0.08}}{{0.02}}$

$= pK_b + 0.602$

$= 3.30 + 0.602= 3.902$

$pH = 10.09$

$[H^+] = 7.99 \times 10^{-11}  \approx 8 \times 10^{-11}\, M$

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