Understanding p-notation
In chemistry, the lowercase letter “p” is a mathematical operator. It is shorthand for “negative logarithm to the base of 10”:
p-notation is used to convert very small, unmanageable numbers (e.g.
Examples uses of p-notation:
| Term | Meaning | Conversion |
|---|---|---|
| pH | Power of Hydrogen | |
| pOH | Power of Hydroxide | |
| pKa | Power of Acid Constant | |
| pKw | Power of Water Constant |
The "Inverse" Rule
Because of the negative sign in the logarithm:
- High p-value = Low actual concentration/constant.
- Low p-value = High actual concentration/constant.
Example: A low pH means high
Definition of pH
pH is a logarithmic scale to express the concentration of hydrogen ions (
Significant Figures Rule
The number of decimal places in the pH value corresponds to the number of significant figures in the concentration. Example:
Strong Acids
A strong acid fully dissociates. Therefore, the concentration of
Case A: Monoprotic Acids (
A monoprotic acid in the form
Ratio of acid to
Example: Find the pH of
Case B: Diprotic Acids (
A diprotic acid in the form
Ratio of acid to
Example: Find the pH of
Strong Bases (Alkalis)
- A strong base fully dissociates to release
not . - We cannot calculate pH directly from the base concentration.
- We must use the Ionic Product of Water (
) to find first.
To find
(Use
The 3-Step Method
- Find
: Look at the concentration of the alkali and use the appropriate mol ratio for (e.g. for or for ). - Find
:
- Calculate pH:
Example 1: Group 1 Alkali (
Find the pH of
- Find
Ratio of is - Find
- Find pH
Example 2: Group 2 Alkali (
Find the pH of
- Find
Ratio of is - Find
- Find pH
Alternative Method: pOH
Some students prefer calculating pOH first.
Both methods are fully accepted in exams.
Proof: