Calculate pH of Buffer Solution

In this video created by Chemistry Guru, Singapore's leading JC Chemistry tuition centre, we want to learn how to determine the pH of a buffer solution via the Henderson-Hasselbach buffer equations.

Usually we are taught that there are 2 buffer equations for different buffer solutions.

We use the acidic buffer equation to determine the pH of an acidic buffer, which is a mixture of weak acid and salt of conjugate base.

calculate pH of buffer solution acidic buffer equation

We use the alkaline buffer equation to determine the pH of an alkaline buffer, which is a mixture of weak base and salt of conjugate acid.

calculate pH of buffer solution alkaline buffer equation

But is it really true that an acidic buffer and alkaline buffer are 2 different types of solutions?

Both are just examples of a mixture of conjugate acid-base pair.

So if they are fundamentally the same type of solution, then the two buffer equations should be interchangeable.

Let's prove this via an example where we have a solution that contains 0.1 moldm-3 CH3COOH and 0.01 moldm-3 CH3COONa, given pKa of CH3COOH is 5.

calculate pH of buffer solution question

A buffer is a mixture of a conjugate acid-base pair and in this case the acid will be CH3COOH while base will be CH3COO-.

calculate pH of buffer solution buffer is mixture of conjugate acid base pair

1. Treat as Acidic Buffer

Since this is a mixture of a weak acid and conjugate base, the most direct way is to treat this as an acidic buffer and use the acidic buffer equation to determine the pH of the solution to be 4.

calculate pH of buffer solution use acidic buffer equation to solve

2. Treat as Alkaline Buffer

We can also use the alkaline buffer equation to calculate the pH but need to take note on the following points:

- pKb is for base CH3COO- which we can determine from pKa of CH3COOH via the formula

pKa + pKb = 14
- base is CH3COO- hence "salt" must be CH3COOH which is the conjugate acid partner of CH3COO-

calculate pH of buffer solution use alkaline buffer equation to solve

Interestingly the pH calculated using the alkaline buffer equation will give the same answer pH = 4.

3. Conclusion

This means that both equations can be used to calculate the pH of any buffer solution.

Personally I'll recommend students to use the acidic buffer equation if Ka is given, and the alkaline buffer equation if Kb is given.

This will allow us to determine pH in the shortest number of steps.

calculate pH of buffer solution use either equation to determine pH of any buffer solution

For the detailed step-by-step discussion on how to calculate the pH of any buffer solution, check out this video!

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

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