Question

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)?

Answer 1

That buffer system has a pH of 4.01

Explanation:

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
(`1.8xx10^(-4))`

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`