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# Which of the following solutions will have $pH$ of 4.74?

(a) 100 mL of 1 M $CH_3COOH$ ($pK_a = 4.74$) at the equivalence point using 1 M NaOH

(b) 50 mL of 1 M $CH_3COONa$ + 25 mL of 1 M HCl

(c) 50 mL of 1 M $CH_3COOH$ + 25 mL of 1 M NaOH

(d) Both (b) and (c)

Answer: 50 mL of 1 M $CH_3COONa$ + 25 mL of 1 M HCl and 50 mL of 1 M $CH_3COOH$ + 25 mL of 1 M NaOH both

In acid base titration, at equivalence point $pH = pK_{in}$

50 mL of 1 M $CH_3COONa$ + 25 mL of 1 M HCl = 25 mL of 1 M $CH_3COOH$ + 25 mL of 1 M NaCl
This is because strong acid HCl will displace weak acid, $CH_3COOH$

In case of an acid buffer solution when
[salt] = [acid]
$pH = pK_a = 4.74$
$\therefore$ Choice (B) is correct.

At mid point of a titration of a weak acid and strong base
$pH = pK_a = 4.74$
$\therefore$ Choice (C) is also correct