A 270.0 mL buffer solution initially contains 3.0×10⁻² M of HCHO₂ and 3.0×10⁻² M of NaCHO₂. To adjust the buffer pH to 4.10, should you add NaOH or HCl to the buffer mixture?

• A. Add HCl to increase the pH.
• B. Add HCl to decrease the pH.
• C. Add NaOH to decrease the pH.
• D. Add NaOH to increase the pH.

Answer: (D)

Verified step by step guidance

1. Identify the components of the buffer solution: HCHO₂ (formic acid) and NaCHO₂ (sodium formate). This is a weak acid and its conjugate base buffer system.
2. Understand the buffer system: The pH of a buffer solution is determined by the Henderson-Hasselbalch equation: ${\mathrm{pH}}_{\mathrm{buffer}}={\mathrm{pK}}_a+\frac{\log }{\frac{{\mathrm{[A}}_{-}}{{\mathrm{[HA}}_{}}}$, where ${\mathrm{pK}}_a$ is the acid dissociation constant of formic acid.
3. Determine the effect of adding NaOH: Adding NaOH will increase the concentration of the conjugate base (NaCHO₂) and decrease the concentration of the weak acid (HCHO₂), thus increasing the ratio $\frac{{\mathrm{[A}}_{-}}{{\mathrm{[HA}}_{}}$ in the Henderson-Hasselbalch equation, leading to an increase in pH.
4. Consider the desired pH change: The problem states that the pH needs to be adjusted to 4.10, which is higher than the initial pH. Therefore, adding NaOH is appropriate as it will increase the pH.
5. Conclude the reasoning: Since adding NaOH increases the pH by increasing the concentration of the conjugate base relative to the weak acid, it is the correct choice to achieve the desired pH of 4.10.