The pKa of the ammonium ion (NH4+) is approximately 9.25. This suggests that the ammonium ion can effectively buffer solutions around this pH value. To determine the pH ranges where it acts as a good buffer, we must look at the options provided:
- a) Between pH 10.25 and 11.25 – This range is above the pKa of 9.25, where the concentration of NH3 (ammonia) would dominate over NH4+. While it can still somewhat buffer, it won’t be effective in resisting changes in pH.
- b) Between pH 7.25 and 8.25 – This range is below the pKa. In this condition, the ammonium ion will predominate, and it can effectively neutralize added acids, making it a good buffering range.
- c) At pH values less than 7.0 – At these lower pH values, NH4+ will dominate, but it is likely that our buffer capacity would not be very effective as it would primarily react with bases, rather than acid, reducing its buffering capabilities.
- d) At pH 4.63 – This is below the pKa; while NH4+ will be present, buffering ability will be limited since it’s too far from the pKa.
In conclusion, the best choice for where the ammonium ion is a good buffer is b) between pH 7.25 and 8.25. This range allows for effective balancing between the concentrations of NH4+ and NH3, allowing the solution to resist changes in pH efficiently.