To determine the highest pH that a buffer system composed of NH4+ (ammonium) and NH3 (ammonia) can effectively buffer, we can use the Henderson-Hasselbalch equation, which is given by:
pH = pKa + log(<[Base]>/<[Acid]>)
Where:
- pH is the pH of the buffer solution.
- pKa is the acid dissociation constant of the weak acid, which is 9.25 for NH4+.
- [Base] is the concentration of the base (NH3).
- [Acid] is the concentration of the acid (NH4+).
Since we are interested in the highest pH and a buffer works best when the concentrations of acid and base are close, we can consider the point where the ratio of [Base] to [Acid] becomes very large (high concentration of NH3 compared to NH4+). In practical terms, we can think about it as:
pH = pKa + 1
This means:
pH = 9.25 + 1 = 10.25
Therefore, considering the options provided, the highest pH at which this buffer system can effectively buffer is:
- a) 10.25
Options b (13.25), c (12.25), and d (11.25) are too high for this buffer system. Therefore, the correct answer is a) 10.25.