A weak acid after treatment with 12ml of 0.1m strong base BOH has a PH of 5. At the end point, the volume of same base required is 26.6ml Ka of acid is 

Option 1)   1.8x10-15

Option 2)   8.12x10-6

Option 3)   1.8x10-6

Option 4)   18.2x10-5

Answers (1)

As learnt in

Buffer Solutions -

The solutions which resist change in pH on dilution or with addition of small amounts of acid or alkali are called buffer solution.  

- wherein

Many body fluids  e.g.  blood

 

 Total milliequivalents of acid = milliequivalents of base

                = 26.6 x 0.1 = 2.66

But the resultant mixture has HA left

The reaction is as follows:

            HA    +    BOH \rightarrow BA    +    H2O

t=0      2.66            12         0              0

t1        1.46             0         1.2           1.2

Therefore for a buffer solution,

pH = pKa+\log \frac{\left [ salt \right ]}{\left [ acid \right ]}

\Rightarrow 5 = pKa+\log \frac{1.2}{1.46}

\Rightarrow \log Ka = -5.86

\Rightarrow Ka = 8.21 \times 10^{-6}

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