Question

One of the ways organisms maintain a stable internal environment (homeostasis) is to use buffers to keep the pH of their body fluids within narrow limits. A biology student mixes 200 mL of a buffered solution with a pH of 4 (solution 'X') with 200 mL of a base (solution 'Y'). After mixing, the pH of the resulting solution (solution 'Z') was found to have risen to 5. Which of the following statements can be made in connection with the experiment described?

Answer 1

Here are valid conclusions you can draw from the experiment and what you cannot determine:

What you can conclude
- Solution Y must have contained base (OH− or something that behaves as a base), because adding it raised the pH from 4 to 5.
- The hydrogen ion concentration fell by a factor of 10 (from 10−4 M to 10−5 M) when X and Y were mixed.
- OH− from Y reacted with the weak acid (HA) in the buffer X, producing its conjugate base (A−). The moles of OH− added equal the increase in moles of A− (i.e., how much HA was converted to A−).
- Using Henderson–Hasselbalch, the ratio [A−]/[HA] increased by a factor of 10 (because pH changed by 1 unit).
- Mixing equal volumes diluted all species twofold, but dilution alone does not change the buffer ratio [A−]/[HA]; the pH change is due to neutralization by added base, not mere dilution.
- The buffer had finite capacity: it resisted a larger pH change but was not able to completely prevent a 1-unit shift.

What you cannot conclude from the information given
- You cannot determine the pKa of the buffer, the absolute concentrations of HA or A−, or the exact concentration or pH of solution Y without more numerical data.