Question

Effects of buffers on pH

"Must show calculations for credit. Be mindful of significant figures."

4. For your samples in Part D. Effects of Buffers on pH, 4A. Which sample (DI water, sodium chloride solution, low pH buffer, or high pH buffer) showed the greatest pH change when acid was added. Why? bi to mo 4B. Which sample (DI water, sodium chloride solution, low pH buffer, or high pH buffer) showed the least pH change when acid was added. Why? . lom DOCX MODEL D 4C. Which sample (DI water, sodium chloride solution, low pH buffer, or high pH buffer) showed the greatest pH change when base was added. Why? 4D. Which sample (DI water, sodium chloride solution, low pH buffer, or high pH buffer) showed the least pH change when base was added. Why? 4E. Are buffers supposed to keep the pH of a solution at 7.0? Why or why not?

Answer 1

4A. NaCl solution will show the greatest pH change.

If you add acid to NaCl solution, the pH will decrease by a factor of "at least" 0.5. This is due to the fact that the pH is not the log of the concentration calculated with respect to the total volume, but the concentration should be calculated with respect to the volume of free water. Therefore, if acid is added to DI water, pH decreases due to acid, but if acid is added to NaCl solution, pH further decreases.

When acid is added to low pH buffer, the H⁺ ions from the added acid combine with excess of conjugate base of acid (of low pH buffer) to form nearly unionized acid (of low pH buffer). Hence, there is no rise in H⁺ ion concentration, and the pH remains unchanged. When acid is added to high pH buffer, the H⁺ ions from the added acid combine with the base (of high pH buffer) to form H₂O and conjugate acid of base (of high pH buffer). Hence, there is no rise in H⁺ ion concentration, and the pH remains unchanged.

4B. Both low pH buffer and high pH buffer will show the least pH change according to the explanation in 4A.

4C. DI water will show the greatest pH change.

If base is added to DI water, pH increases due to base, but if base is added to NaCl solution, pH drops a bit (as explained in 4A).

4D. Both low pH buffer and high pH buffer will show the least pH change.

When base is added to low pH buffer, the OH^- ions from the added base combine with acid (of low pH buffer) to form H₂O and conjugate base of acid (of low pH buffer). Hence, there is no rise in OH^- ion concentration, and the pH remains unchanged. When base is added to high pH buffer, the OH^- ions from the added base combine with excess of conjugate acid of base (of high pH buffer) to form nearly unionized base (of high pH buffer). Hence, there is no rise in OH^- ion concentration, and the pH remains unchanged.

4E. Buffers are not supposed to keep the pH of a solution at 7.0 unless the pH of the solution is 7.0 before a buffer solution is added. Buffers are supposed to keep the pH of a solution "unchanged". This is explained in 4A and 4D (low pH buffer and high pH buffer).