Question

# An acid type indicator, Hln differs in colour from its conjugate base (In−). The human eye is sensitive to colour differences only when the ratio [In−][Hln] is greater than 10 or smaller than 0.1. What would be the minimum change in pH of the solution to observe a complete colour change (Ka = 1.0 × 10−5) ?

A

2

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B

3.25

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C

6

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D

4

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Solution

## The correct option is A 2 Here, to register an observable change the ratio ([In−][Hln] should be greater than 10 or lesser than 10-1.Irrespective of how we effect the change, the magnitude should cross either barrier. For case I pH = pKa + log[salt][Acid] = log(1.0 × 10−5) + log (10) = 5 + 1 = 6 For case II pH = pKa + log[salt][Acid] = log(1.0 × 10−5) + log(0.1) = 5 - 1 = 4 If we inflict a change in pH which is lesser than 6 - 4 = 2, then there is a chance that the human eye might not register it. Thus we need to at the very least, alter the pH by 2.

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