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.
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:
- 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+)
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.
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.
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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