Calculate the pH of a solution that is 0.0190 M in NaOH and 0.0870 M in sodium acetate (K_a for CH_3COOH = 1.75 x 10^-5). What is the pH?

• A. 12.35
• B. 7.00
• C. 9.25
• D. 10.50

Answer: (C)

Verified step by step guidance

1. Identify the nature of the solution: NaOH is a strong base and sodium acetate is a salt of a weak acid (acetic acid, CH3COOH). The solution is a buffer solution due to the presence of the weak acid's conjugate base (acetate ion).
2. Calculate the concentration of hydroxide ions (OH⁻) from the NaOH. Since NaOH is a strong base, it dissociates completely in water. Therefore, the concentration of OH⁻ is equal to the concentration of NaOH, which is 0.0190 M.
3. Use the concentration of OH⁻ to find the pOH of the solution. The formula to calculate pOH is: \( \text{pOH} = -\log[\text{OH}^-] \).
4. Determine the pH from the pOH using the relationship: \( \text{pH} + \text{pOH} = 14 \). Solve for pH by subtracting the pOH from 14.
5. Consider the buffer effect of the acetate ion. Use the Henderson-Hasselbalch equation to adjust the pH: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \( \text{pK}_a = -\log(1.75 \times 10^{-5}) \), [A⁻] is the concentration of acetate ion (0.0870 M), and [HA] is the concentration of acetic acid, which can be assumed to be negligible in this case due to the strong base presence.