Chemistry / Unit 10: Solutions, Acids and Bases
10.5 pH and pOH
Notes
The pH scale measures how acidic a solution is.
- $\text{pH} = -\log[\text{H}^+]$ and $\text{pOH} = -\log[\text{OH}^-]$.
- At 25 °C: $[\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}$ and $\text{pH} + \text{pOH} = 14$.
- pH below 7 is acidic, 7 is neutral, above 7 is basic.
- Each pH unit is a factor of 10: pH 3 has 10 times more $\text{H}^+$ than pH 4.
- For a strong acid, $[\text{H}^+]$ equals the acid concentration: 0.010 M HCl has pH $= -\log(0.010) = 2.00$.
- Going back: $[\text{H}^+] = 10^{-\text{pH}}$.
Practice
What is the pH of a solution with $[\text{OH}^-] = 1.0 \times 10^{-4}$ M?
$\text{pOH} = 4.00$, so $\text{pH} = 14 - 4.00 = 10.00$.
What is the pH of a solution with $[\text{H}^+] = 2.5 \times 10^{-4}$ M?
$\text{pH} = -\log(2.5 \times 10^{-4}) = 3.60$. It is between 3 and 4 because $[\text{H}^+]$ is between $10^{-3}$ and $10^{-4}$.
Solution A has pH 3. Solution B has pH 5. Compared with B, solution A has:
Two pH units is a factor of $10^2 = 100$, and lower pH means more $\text{H}^+$.
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