Question

What is the pH when 25.0 mL of 0.2 M of $C{H}_{3}COOH$ has been titrated with 35.0 mL of 0.1 M $NaOH$ ?
$p{K}_a\left(C{H}_{3}COOH\right)=4.752$
$log2.333=0.368$

• A. 7
• B. 3.45
• C. 5.12
• D. 6.52

Answer 1

The correct option is C 5.12

1) Determine moles of acetic acid and $NaOH$ before mixing:
$C{H}_{3}COOH:\left(0.2mol{L}^{-1}\right)\times \left(0.025L\right)=0.005mol$
$NaOH:\left(0.1mol{L}^{-1}\right)\times \left(0.035L\right)=0.0035mol$

2) Determine moles of acetic acid and sodium acetate after mixing:
$C{H}_{3}COOH:0.005mol-0.0035mol=0.0015mol$
$C{H}_{3}COONa:0.0035mol$

3) Use the Henderson - Hasselbalch Equation:
$pH=p{K}_a+log\left[\frac{\left[\text{conjugate base}\right]}{\left[\text{acid}\right]}\right]$
$pH=p{K}_a+log\left[\frac{\left[C{H}_{3}COONa\right]}{\left[C{H}_{3}COOH\right]}\right]$
$pH=4.752+log\frac{\frac{0.0035}{0.060}}{\frac{0.0015}{0.060}}$
$pH=4.752+log2.333$
$pH=4.752+0.368=5.120$