Calculate Ka of acid.

Question

A weak acid HA after treatmeant with 12 mL of 0.1 M strong base BOH has a pH of 5. At the end point, the volume of same base required is 26.6 mL. Calculate

K_a of acid.

Answer 1

Solution..........

Comments

At first, why did you wrote 'For complete neutralisation'?

2nd, in partial neutralization, why you took initial volume of HA to 26.6 mL?

I didn't understand, please help me.

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In this question you have mentioned endpoint, that means the whole reaction has taken place .  For complete reaction the reactant must not be left, only the product should be there. So complete neutralisation is counted for the whole.
Whereas, The initial volume of base is given  and total utilised base is also given. by formula the eq of acid = eq of base we can assume total used acid(HA) in the solution as 26.6 ml. So we take all the required acid at the begning of the experiment and what remained un-utilised is left at final stage (12.6ml left here).