ChemistryGeneral ChemistryHigh school

Acids, Bases and pH

pH, pOH and hydrogen ion concentration.

Acids donate hydrogen ions (protons) and bases accept them, in the Brønsted-Lowry picture. Strength is about how completely a substance ionises in water, not how concentrated it is: hydrochloric acid ionises fully, while acetic acid in vinegar ionises only slightly.

The pH scale measures hydrogen-ion concentration logarithmically, $\text{pH} = -\log[\text{H}^+]$ . Each pH unit is a factor of ten, so pH 3 is ten times more acidic than pH 4. At $25^\circ$ C, $\text{pH} + \text{pOH} = 14$ , and neutral water has pH 7.

Key terms

pH: Negative base-10 logarithm of hydrogen ion concentration.
$\text{pH} = -\log_{10}[\text{H}^+]$

Key ideas

$\text{pH} = -\log[\text{H}^+]$ ; $[\text{H}^+] = 10^{-\text{pH}}$ .
For strong acids $[\text{H}^+]$ equals the acid concentration (per ionisable proton).
Neutralisation: acid + base $\to$ salt + water.
Titration at equivalence: moles of $\text{H}^+$ = moles of $\text{OH}^-$ .

Worked example

Find the pH of $0.001$ M hydrochloric acid.

HCl is strong, so $[\text{H}^+] = 0.001 = 10^{-3}$ M.
$\text{pH} = -\log(10^{-3}) = 3$ .
pOH $= 14 - 3 = 11$ .

Answer: $\text{pH} = 3$

Common mistakes

Treating strong as meaning concentrated.
Forgetting the minus sign in the pH formula.
Assuming a tenfold dilution changes pH by ten units rather than one.

Practice questions

Try each one, then open the answer. Numerical answers on this page are verified by an automated calculation check.

1.pH of a solution with $[\text{H}^+] = 10^{-5}$ M.
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$5$
2.pOH of a solution with pH 9.
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$5$
3.Volume (mL) of 0.1 M NaOH to neutralise 25 mL of 0.2 M HCl.
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$50$

Key terms

Key ideas

Worked example

Common mistakes

Practice questions

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