pH of a mixture containing 0.10 M ${\mathrm{X}}^{-}$ and 0.20 M HX is   $\left[{\mathrm{pK}}_{\mathrm{b}}\left({\mathrm{X}}^{-}\right) = 4\right]$

• A

  4 + log 2

• B

  4 - log 2

• C

  10 + log 2

• D

  10 - log 2