HA的pka=4.3,原溶液ph=pka+lg[NaA]/[HA]=5.5 ,5.5=4.3+lg[NaA]/0.25,解得[NaA]=3.96molL-,100ml原溶液中,nHA=0.1x0.25=0.025mol,nNaA=0.1x3.96=0.396mol,,加入5x10^-3molNaOH与HA负形成0.005molNaA,剩余0.02molHA,新溶液中nHA=0.02mol,nNaA=0.396+0.005=0.401mol,ph=4.3+lg0.401/0.02=5.6