pH of Weak Bases

Whenever these questions are asked, they are worth Excellence. This is because there are six key steps to remember, and they need to be remembered in order!

1. Write out the equation for the base acting as a an alkaline solution.
2. Write a K_b expression.
3. Calculate K_b from K_a
4. Calculate [OH^-] from the K_b expression (assuming [HB⁺] = [OH^-]
5. Calculate [H₃O⁺] from [OH^-], using K_W
6. Calculate pH

This video is very long, so use it to work through an example, rather than trying to learn it all in one go:

pH of Weak Acids

Weak acids only partially dissociate, so how do we calculate their pH?

We were encouraged to work through pp180-181 in Continuing Chemistry to check whether or not we have understood this.

pH of Strong Acids and Bases

This is just a recap of last year, but these skills are vital for moving forward in this topic:

Here is the concept being taught to a Year 12 class:

Acids, Bases and Salts

We started with a recap of the Bronsted-Lowry definitions of acids, bases and amphiprotic species:

Then, we looked at the pH of some salts, comparing them to a control of NaCl (known to have a pH of 7.0):

We need to write chemical equations for the ions to justify the observed pH values (alkaline or acidic):

For example:

HCO₃^- + H₂O <=> H₂CO₃ + OH^-

This equation shows an increase in [OH^-], which is expected as the pH > 7.0