Study guides › C/P › General Chemistry
Acids & Bases
Acid-base chemistry runs through general chemistry, organic chemistry, and physiology. You will learn the three definitions, calculate pH for strong and weak acids and bases, predict the pH of salts, and handle polyprotic acids.
Study this chapter in PrepMed (free)
What this chapter covers
- Acid-Base Definitions & Conjugate Pairs. The MCAT expects you to switch between three definitions of acid and base without hesitation, and to spot conjugate pairs instantly.
- pH, pOH & Strong Acids and Bases. Strong acid and base pH problems are pure logarithm arithmetic, and the exam loves the traps hidden in them.
- Weak Acids & Bases: Ka, Kb and pKa. Weak acid and base equilibria give the MCAT its favorite approximation: [H⁺] ≈ √(Ka·C).
- Salts: Acidic, Basic or Neutral?. Salt questions ask which ion reacts with water. Only ions descended from weak acids or weak bases do.
- Polyprotic Acids & Amphoteric Ions. Polyprotic acids lose protons one at a time, and every step has its own Ka. That shapes pH, buffers, and amino acids.
Key points to know
- Brønsted-Lowry acid = proton donor; base = proton acceptor
- Lewis acid = electron-pair acceptor; Lewis base = electron-pair donor
- Kw = 1.0 × 10⁻¹⁴ at 25 °C; pH + pOH = 14
- Each pH unit is a tenfold change in [H⁺]
- Ka = [H⁺][A⁻]/[HA]; smaller pKa means stronger acid
- Ka × Kb = Kw, so pKa + pKb = 14
- Ions from strong acids/bases (Na⁺, K⁺, Cl⁻, NO₃⁻) do not affect pH
- Anions of weak acids are basic; NH₄⁺ and highly charged metal cations are acidic
Try a practice question
Removing one proton from the dihydrogen phosphate ion, H₂PO₄⁻, gives which species?
- H₃PO₄
- HPO₄²⁻
- PO₄³⁻
- OH⁻
Show the answer
B. Remove one proton from H₂PO₄⁻ to get HPO₄²⁻. H₃PO₄ is its conjugate acid (one proton added), and PO₄³⁻ is two protons away.
Common questions
What does this chapter cover?
Acids & Bases covers Acid-Base Definitions & Conjugate Pairs, pH, pOH & Strong Acids and Bases, Weak Acids & Bases: Ka, Kb and pKa, Salts: Acidic, Basic or Neutral?, Polyprotic Acids & Amphoteric Ions.
What should I know first?
Brønsted-Lowry acid = proton donor; base = proton acceptor Lewis acid = electron-pair acceptor; Lewis base = electron-pair donor Kw = 1.0 × 10⁻¹⁴ at 25 °C; pH + pOH = 14
24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.