the main formulas are pH=pKa + log(Base/Acid) and pKa = -log(Ka)
so firstly, we must find the value of pKa,
Ka=6.37 x 10 ^-5, and then logKa= log (6.37 . 10^-5)= -9.66, so -logKa= +9.66=Ka
next let's find log(Base/Acid)
for that the concentration of NaOH is [NaOH] = 500.0 x 0.110 / 500+535 =0.053M, the concentration of the Acid is [Acid] =535*0.25 / 500+535 =0.12M, so its difference is [Acid]-[NaOH] = 0.12-0.053=0.07
so pH=pKa + log(Base/Acid)= 9.66 + log(0.053 / 0.07)= 9.66-0.36=9.29
so pH=9.29.
