How do you answer a questions on how a buffer solution works?

This question would be a 5/6 mark question- marks are displayed throughout

You always start by defining what a buffer solution is: A buffer solution is a system that minimises a change in pH on the addition of a small amount of acid or a base. (1) Then you would write out the equilibrium of the buffer solution, for example, CH3COOH (=) CH3COO+ H+ (2). The conjugate base in this solution is CH3COOand the weak acid is the CH3COOH.

Then you would explain the effect of adding a small amount of acid to the buffer solution:

The conjugate base (CH3COO-) removes the added acid. When an acid is added the [H+] increases and these Hreact with the conjugate base (3). The equilibrium shifts to the left (4), removing most of the Hions.

The weak acid (CH3COOH) removes the added alkali. When an alkali is added the [OH-] increases. The small amount of Hpresent reacts with the OHto form H2O (5). The weak acid dissociates, causing the equilibrium to shift to the right (6) to restore most of the H+ ions. 

I would the attach an example exam question which they can complete. 

AR
Answered by Alice R. Chemistry tutor

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