`20 mL` of a weak monobasic acid `(HA)` requires `20 mL 0.2 M NaOH` for complete titration. If `pH` of solution upon addition of `10 mL` of this alkali to `25mL` of the above solution of `HA` is `5.8`. The `pK_(a)` of the weak acid is

Options

A. `6.1`
B. `5.8`
C. `5.98`
D. `5.58`

Answer

C

Explanation

meq. Of acid=meq. Of base `implies 20xxM=20xx0.2=4`
Molarity of `HA=0.2 M`
`underset(3)underset(5)(HA)+underset(-)underset(2)(OH) rarr underset(2)()(A^(-))+underset(m"mole")underset(m"mole")(H_(2)O)`
`:. pH=pK_(a)+log``([A^(-)])/([HA])`
`implies 5.8=pK_(a)+log``((2)/(3))`
`implies pK_(a)=5.98`