If very small amount of HPh (phenolphthalein) added to 0.1 M aque… - Vidyadip

MCQ

If very small amount of $HPh$ (phenolphthalein) added to $0.1\ M$ aqueous solution of $CH_3 COONa$. The fraction of indicator that exists in colourless form is Given

$[K_b\,(CH_3COO^-) =10^{-9},pK_a\,(HPh)=9.6,log\,2 =0.3]$

A
$0.2$
✓
$0.8$
C
$0.5$
D
$0.4$

✓

Answer

Correct option: B.

$0.8$

b
${\text{pH}} = \frac{1}{2}({\text{pKw}} + {\text{pKa}} + \log \,{\text{C}})$

$ = \frac{1}{2}(14 + 5 - 1) = 9$

${\text{pH}} = {\text{pKa}} + \log \,\frac{{{\text{I}}{{\text{n}}^ - }}}{{{\text{HIn}}}}$

$\log \,\frac{{{\text{HIn}}}}{{{\text{I}}{{\text{n}}^ - }}} = 0.6 = 2 \times 0.3 = 2\,\log \,2$

$\frac{\mathrm{HIn}}{\mathrm{In}^{-}}=4, \mathrm{HIn}+\mathrm{In}^{-}=1.0, \mathrm{HIn}=0.80$

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