a. At what `pH` does indicator change colour if the indicator is a weak acid with `K_(Ind) = 4.0 xx 10^(-4)`. b. For which of the following neutralisation would the indicator be useful? i. `HCl +NaOH` ii. `CH_(3)COOH +NaOH` iii. `HCl +NH_(3)` c. Name the indicators which can be used for such titration.

a. At what `pH` does indicator change colour if the indicator is a wea

1.	a. At what `pH` does indicator change colour if the indicator is a weak acid with `K_(Ind) = 4.0 xx 10^(-4)`. b. For which of the following neutralisation would the indicator be useful? i. `HCl +NaOH` ii. `CH_(3)COOH +NaOH` iii. `HCl +NH_(3)` c. Name the indicators which can be used for such titration.
Answer» Correct Answer - A::B::C::D i. The mid-point of the colour change range of an indicator is the point at which its acid and conjugate base forms are present in equal concnetration. `pH = pK_(Ind) =- (log K_(Ind))` `=- log (4 xx 10^(-4))` `= - log 2^(2) + 4` `=- 0.6 + 4 = 3.4` ii. `pH` range of the indicator is `3.4` and is suitable for the titration of `S_(A)//W_(B).(HC1 + NH_(3))`. iii. The `pH` at the end point lies between `3.0` and `6.0`. For such titrations, methy`1` ornage `(pH` range `3.1 - 4.5)`, methy`1` red `(pH` range `4.2 - 6.3)`, and bromophenl blue `(3.1 - 4.6)` are thus suitable indicators.

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