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How to Calculate pH at Equivalence Point

In this JC2 webinar we want to learn how to calculate the pH at equivalence point.

Question: Determine the pH of resultant solution when 25.0 cm³ of 0.1 moldm^-3 NaOH is added to 25.0 cm³ of 0.1 moldm^-3 CH₃COOH. Assume K_a of CH₃COOH is 10^-5 moldm^-3.

The ICE table is very useful to determine the resultant solution of an acid-base reaction.

calculate eqv pt ICE table

The resultant solution at complete neutralisation is the salt solution of CH₃COONa.

Therefore the pH at equivalence point will just be the pH of this salt solution.

We need the concept of salt hydrolysis to deduce the nature of this salt.

calculate eqv pt salt hydrolysis

If you are interested to know about salt hydrolysis in detail, check out this video that I've previously done on how to deduce the nature of salts.

Sodium ethanoate is alkaline since CH₃COO^- is the conjugate base of weak acid CH₃COOH.

Hence it will dissociate in water to give OH^- and make the solution alkaline.

calculate eqv pt explain why eqv point is alkaline

Now that we know the equivalence point is alkaline, how do we calculate its pH?

Notice we have established the following relationship:

equivalence point → salt → conjugate base → weak base

Therefore we can use the formula of weak base to calculate the OH^- concentration at this equivalence point.

calculate eqv pt calculate using weak base equation

K_b is not given but we can calculate it easily from ionic product of water K_w and K_a via the formula K_w = K_a.K_b

calculate eqv pt determine Kb

Concentration of CH₃COO^- can also be easily calculated, keeping in mind the total volume is 50cm³.

calculate eqv pt determine conc of CH3COO

We can now substitute these back into the weak base formula to find OH^- concentration, and eventually the pH at this equivalence point.

calculate eqv pt determine hydroxide concentration and pH

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

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How to Calculate pH at Equivalence Point