Chemistry Content: Solutions, Acids and Bases, and Electrochemistry
This domain tests concentration, solubility, acid-base behavior, pH, buffers, titration, redox, and electrochemical cells. Items may ask you to calculate pH or cell potential, interpret a titration curve, or identify oxidation and reduction half-reactions.
Core rules and concepts
- Molarity M = moles solute / liters solution. Dilution uses M₁V₁ = M₂V₂ because moles of solute are conserved.
- Strong acids and bases ionize completely; weak acids and bases partially ionize and require Ka or Kb for pH.
- pH = −log[H⁺]; pOH = −log[OH⁻]; pH + pOH = 14 at 25 °C.
- A buffer resists pH change when it contains a weak acid and its conjugate base (or weak base and conjugate acid).
- In redox, oxidation is loss of electrons at the anode; reduction is gain at the cathode. In a galvanic cell, E°cell = E°cathode − E°anode and must be positive for a spontaneous reaction.
Worked example
Find the pH of 0.010 M HCl. HCl is a strong acid, so [H⁺] = 0.010 M. pH = −log(0.010) = 2.00. If this solution is diluted tenfold to 0.0010 M, pH = 3.00. The dilution changes concentration but not the number of moles of HCl initially present.
Traps that cost marks
Do not use pH = −log(concentration) for a weak acid without finding [H⁺] from Ka. Do not confuse pH with pOH. In a galvanic cell, the anode is negative and oxidation occurs there; electrons flow from anode to cathode in the external circuit. Do not forget that dilution changes pH but not the moles of solute. In titration, the equivalence point is not always pH 7.