Final answer:
To calculate the pH when it is within 2 units (+/-) of pKa, you need to use the Henderson-Hasselbalch equation. By plugging in the values of pKa and the ratio of acid to its conjugate base or base to its conjugate acid, you can calculate the pH of the solution.
Step-by-step explanation:
If the pH is within 2 units (+/-) of pKa, you are calculating the ratio of acid to its conjugate base or the ratio of base to its conjugate acid. This calculation is done using the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation is expressed as pH = pKa + log([base]/[acid]). By plugging in the values of pKa and the ratio, you can calculate the pH of the solution. For example, if the pKa is 4 and the ratio is 10, the pH would be 6 due to the logarithmic relationship between the ratio and pH.