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Basic Buffer Action

A 40mL samp...

Question

A $40 m L$ sample of $0.1 M$ solution of nitric acid is added to $20 m L$ of $0.3 M$ aqueous ammonia. What is the pH of the resulting solution? Given, $K_{b}$ for ammonia $1.8 \times 10^{- 5}$ .

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Solution

$H N O_{3} + N H_{4} O H ⇌ N H_{4} N O_{3} + H_{2} O$
$4 m i l l i m o l$ $6 m i l l i m o l$
$2 m i l l i m o l$ $4 m i l l i m o l$
(Weak base) and (salt of weak base and strong acid) will form basic buffer.
So $p O H = {p K}_{b} + log \frac{[S a l t]}{[B a s e]}$
$p O H = 5 - log (1.8) + log \frac{4}{2}$
$p O H = 4.96$
As we know that $p H + p O H = 14$
$p H = 9.04$

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