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Question $33$

Consider the buffers below composed of the weak acid HA and its conjugate base $A^{-}.$ In which case does the relation $pH=p{K}_n$ boid?

$0\mathrm{.}10MH{A}^{**}0\mathrm{.}10M{A}^{-}$

$0\mathrm{.}10MH{A}^{-0\mathrm{.}90}M{A}^{-}$

$0\mathrm{.}75$MHA;$0\mathrm{.}50MA$

$0\mathrm{.}90$MHA;$0\mathrm{.}10MA=$