Work Step by Step
By the Henderson-Hasselbach Equation: $pH=pKa+\log([A^-]/[HA])$ $[A^-]=[OH^-]=1.00\ M\times V/(V+250.0)$ $[HA]\approx0.5\ M\times 250.0\ mL/(V+250.0)\ mL$ $4.5=4.76+\log(V/125)$ $V=68.7\ mL$
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