MCAT® ExamChemical and Physical Foundations of Biological SystemsEasy
A student measures the pH of a 0.010 M solution of an unknown acid and finds it to be 3.00. Based on this measurement, what can be concluded about the unknown acid?
- AIt is a weak acid that partially dissociates in water.
- BIt is a strong acid that completely dissociates in water.
- CIt is a very concentrated solution of a very weak acid.
- DIt is a neutral solution, as pH 3.00 is close to 7.00.
Show answer & explanationAnswer & explanation
Correct answer: A. It is a weak acid that partially dissociates in water.
For a 0.010 M strong acid, the [H⁺] would be 0.010 M, resulting in a pH of -log(0.010) = 2.00. Since the measured pH is 3.00, meaning [H⁺] = 10⁻³ M = 0.001 M, the acid has not completely dissociated. This indicates it is a weak acid.
Why the other options are wrong
- B. If it were a strong acid, pH would be 2.00, not 3.00.
- C. While it is a weak acid, the pH of 3.00 indicates a [H⁺] of 0.001 M, which is less than the initial acid concentration of 0.010 M, meaning it's not 'very concentrated' such that it would hide a very weak dissociation, but rather simply a weak acid.
- D. A pH of 3.00 is acidic, not neutral.
Strong vs. Weak Acids/Bases
Strong acids and bases completely dissociate in water, while weak acids and bases only partially dissociate, establishing an equilibrium.
- Strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄.
- Strong bases: Group 1 hydroxides (LiOH, NaOH, KOH, etc.), Group 2 hydroxides (Ca(OH)₂, Sr(OH)₂, Ba(OH)₂).
- Weak acids have a K_a < 1; weak bases have a K_b < 1.
- The pH of a weak acid solution depends on its K_a and initial concentration, and is calculated using equilibrium expressions.
Memory trick: Compare measured pH to strong acid pH; if higher, it's weak!