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  4. The ionisation constant of an acid-base indicator (a weak acid) is 1.0 × 10-6. The ionized formed of the indicator is red and unionized form is blue. The pH change required to alter the colour of indicator from 80 % red is

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Q. The ionisation constant of an acid-base indicator (a weak acid) is $1.0 \times 10^{-6}$. The ionized formed of the indicator is red and unionized form is blue. The $pH$ change required to alter the colour of indicator from $80 \%$ red is

NEETNEET 2022Equilibrium

Solution:

$HIn \rightleftharpoons H ^{\oplus}+\operatorname{Ind}^{\Theta}$
$pH =p K_{\text {Ind }}+\log \frac{\text { Ind }^{\Theta}}{[ HIn ]}$
$pH _1=p K_{\text {Ind }}+\log \frac{20}{80}=p K_{\text {Ind }}=2 \log 2$
$pH _2=p K_{\text {Ind }}+\log \frac{80}{20}=p K_{\text {Ind }}+2 \log 2$
$\Delta( pH )= pH _2= pH _1=4 \log 2=4 \times 0.3=1.2$