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Question

The pH of a buffer solution is 5. It consists of a weak acid HA and it's conjugate base A−. PKa of HA is 4.699. In which of the following conc. HA and A− are present in the solution?

A
100 ml of 0.1 M HA and 200 ml of 0.1 M A
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B
200 ml of 0.1 M HA and 100 ml of 0.1 M A
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C
100 ml of 0.1 M HA and 200 ml of 0.2 M A
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D
200 ml of 0.1 M HA and 100 ml of 0.2 M A
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Solution

The correct option is A 100 ml of 0.1 M HA and 200 ml of 0.1 M A
The expression for the pH of the acidic buffer solution is as given below.

pH=pKa+logSalt][Acid]

pKa=4.699, pH=5

Substitute values in the above equation.
5=4.699+log[Salt][Acid]

Hence, log[Salt][Acid]=0.3010 or [Salt][Acid]=2

In option A, [Salt]=0.2×0.1=0.02 M and [Acid]=0.1×0.1=0.01 M

Hence, [Salt][Acid]=0.20.1=2.

Hence, the correct option is A

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