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)?

1 Answer
Aug 11, 2016

That buffer system has a pH of 4.01


Before I introduce Henderson-Hasselbalch's equation, we should identify the acid and base. Formate #(HCO_2^(-))# is the conjugate base of the acid, formic acid, #(HCO_2H)#. A conjugate base has one less proton #(H^+)#than the acid you started with.

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:

#pKa= -log(1.8xx10^(-4))# = 3.74

Thus, your pKa is 3.74

Next, we can obtain the concentration of base and concentration of acid from the question.

[#HCO_2^(-)#] = .50 M [#HCO_2H#] =.27 M

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:

#pH = 3.74 + log( [0.50]/[0.27])#

#pH = 4.01#