How would you use the Henderson-Hasselbalch equation to calculate the pH of a buffer solution that is 0.27 M in formic acid (#HCO_2H#) and 0.50 M in sodium formate (HCO_2Na)?
That buffer system has a pH of 4.01
Before I introduce Henderson-Hasselbalch's equation, we should identify the acid and base. Formate
Now, we can use this equation:
For this type of question we need a pKa, which is not given in your question. Therefore, I Googled the Ka and it is
To obtain the pKa from the Ka value, all we have to do is take the
negative logarithm (-log) of the Ka:
Thus, your pKa is 3.74
Next, we can obtain the concentration of base and concentration of acid from the question.
We're not really concerned with the sodium cation that attached to the formate ion because it's a spectator ion and it has no effect on the buffer system.
Now, we have all of the information to determine the pH. Let's plug our values into the equation: