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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 × 105) ?


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 × 105) + log (10) = 5 + 1 = 6

For case II

pH = pKa + log[salt][Acid] = log(1.0 × 105) + 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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