pH Formula
The Formula
\text{pH} = -\log[\text{H}^+]
When to use: A number that tells you: acid (low), neutral (7), base (high).
Quick Example
Lemon juice: pH ~2 (acidic). Pure water: pH 7 (neutral). Bleach: pH ~13 (strongly basic).
What This Formula Means
A logarithmic scale from 0 to 14 that measures the concentration of hydrogen ions in a solution, indicating how acidic or basic it is.
A number that tells you: acid (low), neutral (7), base (high).
Worked Examples
Example 1
easyCalculate the pH of a solution with [\text{H}^+] = 1.0 \times 10^{-4}\,\text{M}.
Solution
- 1 Recall the pH formula: \text{pH} = -\log[\text{H}^+], where [\text{H}^+] is the hydrogen ion concentration in mol/L.
- 2 Identify the given value: [\text{H}^+] = 1.0 \times 10^{-4}\,\text{mol/L}.
- 3 Apply the formula: \text{pH} = -\log(1.0 \times 10^{-4}) = -(-4) = 4
Answer
\text{pH} = 4
The pH scale is logarithmic — each unit change in pH represents a tenfold change in hydrogen ion concentration. A pH of 4 means the solution is acidic.
Example 2
mediumA solution has pH = 9. Calculate [\text{H}^+] and [\text{OH}^-].
Why This Formula Matters
pH affects everything from enzyme activity to environmental health.
Frequently Asked Questions
What is the pH formula?
A logarithmic scale from 0 to 14 that measures the concentration of hydrogen ions in a solution, indicating how acidic or basic it is.
How do you use the pH formula?
A number that tells you: acid (low), neutral (7), base (high).
Why is the pH formula important in Chemistry?
pH affects everything from enzyme activity to environmental health.
What do students get wrong about pH?
pH 3 is 10\times more acidic than pH 4, not just 'one more.'
What should I learn before the pH formula?
Before studying the pH formula, you should understand: acid, base, concentration.