Calculate pH of Salt Solution

In this video we want to deduce the nature and calculate the pH of sodium ethanoate solution.

Let's take a look at this question to calculate the pH of 0.500 mol dm-3 sodium ethanoate given Ka of ethanoic acid is 1.8 x 10-5 mol dm-3.

calculate pH of salt solution question

Salt Hydrolysis

First we will have to deduce the nature of sodium ethanoate, which is a concept of salt hydrolysis.

I have a previous video on salt hydrolysis so if you are interested do check it out.

To summarise this is the concept that we need to know:

calculate pH of salt solution conjugate acid base concept

- weak acid (eg CH3COOH) will dissociate to give weak conjugate base (CH3COO-)
- weak base (eg NH3) will dissociate to give weak conjugate acid (NH4+)
- strong acid (eg HCl) will dissociate to give neutral counter ion (Cl-)
- strong base (eg NaOH) will dissociate to give neutral counter ion (Na+)

calculate pH of salt solution deduce nature of sodium ethanoate

So for sodium ethanoate, we can deduce that CH3COO- is the conjugate base of weak acid CH3COOH, hence will dissociate in solution to give OH- and pH will be greater than 7.

calculate pH of salt solution explain why sodium ethanoate solution is alkaline

Notice the dissociation is reversible as conjugate base ethanoate is a weak base.

So for calculation of pH we can use the formula of weak base to determine the OH- concentration and eventually solve for pH.

calculate pH of salt solution calculate pH of sodium ethanoate

Kb of ethanoate is not known but we can determine it from Ka of ethanoic acid using the formula:

Ka x Kb = Kw (for conjugate acid-base pair)

It is important to note that the method to find pH of a salt solution is to deduce that one of the ions is a conjugate base (in this example), which is also a weak base.

So the method to find the pH of sodium ethanoate is nothing more than finding the pH of a weak base.

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

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