Explain how pH changes can be minimised using a mixture of a weak acid and it's conjugate base

This mixture of a weak acid (HA) and it's conjugate base (A-) acts as a buffer. A buffer is a solution with a highly stable pH. On addition of small amounts of either a strong acid or a strong base, this buffer will resist changes in pH.

Before addition:
The acid and it's conjugate base may react with each other, but this will result in no net change as both acid and base are reformed in the reaction. 

HA + A--> A+ HA

Both the acid and it's conjugate base are weak so rarely react with the water present. 

As a result of both of these, the weak acid and it's conjugate base will remain in high concentration in the solution.

On addition of strong base:
As the base added is strong, it will react with the weak acid. The equation below demonstrates how the Hfrom the weak acid is donated to the -OH group from the base. 

 -OH + HA --> A+ H2O

As the -OH group is used up by the reaction, the pH will only change minimally.

On addition of strong acid:
The Hfrom the strong acid will react with the weak base to form the weak acid HA. 

H+ + A-  --> HA

The additional H+ contributed by adding the strong acid is mopped up by the weak base. Therefore the pH will only change minimally

Answered by Cora T. Chemistry tutor

3957 Views

See similar Chemistry A Level tutors

Related Chemistry A Level answers

All answers ▸

What is the difference between a nucleophile and an electrophile?


Providing reasoning, what is the trend in the atomic radius of row 3 elements across the periodic table?


What are 'Rate equations' and why are they useful?


How do I predict the shape and bond angle of an molecule?


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

© MyTutorWeb Ltd 2013–2025

Terms & Conditions|Privacy Policy
Cookie Preferences