Answer the following questions (assuming that all activity coefficients are equa
ID: 554072 • Letter: A
Question
Answer the following questions (assuming that all activity coefficients are equal to 1), including showing all working: (2a). What salt would you obtain from a neutralisation reaction between of Benzoic acid with ammonia? (1 mark) b) State whether an aqueous solution of the salt (from your answer to (a)) would be neutral, acidic or alkaline. Briefly explain your reasoning behind your answer. (3 marks) clState Le Châtelier's principle and discuss its relevance to buffer solutions. (4 marks) d)Estimate the pH of an aqueous buffer solution containing 0.2M of Benzoic acid and 0.2M of its It. (2 marks)Explanation / Answer
a) the reaction of benzoic acid with ammonia will be:
C6H5COOH + NH3 ---> C6H5-NH4+
It will give salt ammonium benzoate
b) the Ka for benzoic acid = 6.3 X 10-5 and Kb for NH3 = 1.8 X 10-5
so the strength of acid is slightly stronger than the base, hence the solution will be acidic [pKa < pKb].
c)Le-Chatelier's principle is used in relevance to buffer solutions.
The buffer solution works on the Le-Chatelier's principle for resisting change in pH.
For example if we have a buffer of NH3 and NH4Cl
Then on addition of H+
The ammonia will react with H+ to neutralize its effect and will generate more salt
On addition of OH-. the salt will react with hydorixde ion to give more ammonia and will neutralize its effect
In both the conditions the equilibrium is shifted in other direction where the addition is made and hence the pH is maintained
d) the pKa of benzoic acid = 4.20
pH of buffer solution is calcualted as
pH = pKa + log [salt] / [acid]
pH = 4.20 + log [0.2 / 0.2] = 4.20