How does a buffer actually work?

[Kaushik]

TL;DR Summary

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Buffer is a solution of weak acid and its conjugate base which resists the change in pH when strong acid/base is added to the solution.

But how does it work?

Consider,

HA + H2O ⇄ H3O⁺ + A^-

When we add strong acid, the H+ from the strong acid reacts with the conjugate base A- to for HA. Hence, the concentration of HA increases while the concentration of A- decreases. According to Le Chatelier's principle the equilibrium reaction shifts to the right. So then won't the H3O+ concentration increase further? We want it to shift to the left right?

When we add strong base, the OH- reacts with H+ from the weak acid to form H2O. So concentration of HA decreases while the concentration of A- increases hence the reaction should shift towards the left, am I correct? It was given it shifts towards the right. How?Thanks!

 

[Borek]

When we add strong acid, the H+ from the strong acid reacts with the conjugate base A- to for HA. Hence, the concentration of HA increases while the concentration of A- decreases. According to Le Chatelier's principle the equilibrium reaction shifts to the right.

No, it shifts to the left and stops there.