Calculate pH Change of Buffer when Hydroxide Added

In this video we want to learn how to calculate pH change of buffer when small amounts of hydroxide is added.

230109 question 2a

This solution is a mixture of weak acid ethanoic acid and conjugate base ethanoate.

Therefore this solution is a buffer solution.

230109 components of buffer

Check out this video for an more indepth discussion of how to identify buffer solution.

Hence we can use the buffer equation to calculate its pH.

230109 calculate pH of buffer

Learn how to calculate pH of buffer solution here.

230109 question 2b

Next 0.050 mol of sodium hydroxide is added to this solution.

Since the function of a buffer is to maintain pH, it must react away this added hydroxide.

Therefore it will be the job of ethanoic acid to remove this amount of OH-.

230109 function of buffer

We can use the ICE table to determine the resultant solution.

230109 determine resultant solution

Notice the final solution is still a mixture of CH3COOH and CH3COO-, hence it remains a buffer.

So we can use the same buffer equation to determine its final pH and the change in pH.

230109 calculate change in pH

Notice when small and limiting amount of OH- is added, the change in pH is very small.

This is consistent with the definition of a buffer solution, where pH is maintained when small amounts of H+ or OH- is added to it.

Topic: Buffer Solution and Titration Curve, Physical Chemistry, A Level Chemistry, Singapore

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