In the titration of solution of a weak acid HA and NaOH, the pH is 5.0 after 10 mL of NaOH solution has been added and 5.60 after 20 mL NaOH has been added.
What is the value of `pK_(a)` for HA?

Explanation

Let the mormality of `HX` and `NaOH = 1`.
Let `x mEq` or mmol of `HX` is present initially.
When `pH = 5.8`.
`["Salt"] = 10 xx 1 = 10 mmol`
`["Acid"] = (x - 10) mmol`
When `pH = 6.4`
`["Salt"] = 20 xx 1 = 20 mmol`
`["Acid"] = (x - 20) mmol`
`5.8 = pK_(a) +log (10//x - 10) ..(i)`
`6.4 = pK_(a) +log (20//x - 20)...(ii)`
SOlving equaitons (i) and (ii), calculate the value of `x` and `pK_(a)`.
`:. pK_(a) = 6.1`