General Chemistry for the MCAT Exam

Common failure modes

These are the patterns that cause most candidates to lose marks on this topic. Recognising them in advance is half the work.

• !Using ΔG = ΔH − TΔS without checking signs — a common source of wrong answers on spontaneity questions
• !Forgetting that a buffer's pH ≠ pKa unless [acid] = [conjugate base]
• !Confusing galvanic (spontaneous, ΔG < 0) and electrolytic (non-spontaneous, ΔG > 0) cells
• !Not converting to standard conditions before applying ΔG° = −nFE°
• !Struggling with limiting-reagent stoichiometry embedded inside a multi-paragraph passage

Study tips

• 1Memorize ΔG = ΔH − TΔS and the four sign combinations — know which conditions make a reaction always, never, or temperature-dependent spontaneous.
• 2Practice Henderson-Hasselbalch calculations daily. Master the rule: within one pH unit of pKa, the buffer is effective; at pKa, [HA] = [A−].
• 3Draw a galvanic cell from scratch — label anode (oxidation, negative), cathode (reduction, positive), electron and ion flow directions.
• 4Review orbital hybridization and VSEPR geometry — these appear in C/P as quick conceptual questions between quantitative passages.

Sample MCAT General Chemistry questions

These sample items mirror the format and difficulty of real MCAT questions. Practice with thousands more on the free Koydo question bank.

1. 1

   A buffer is prepared by mixing acetic acid (pKa 4.76) and sodium acetate. The final pH of the solution is 5.06. The ratio of acetate to acetic acid in this buffer is approximately:

   • A1 : 1
   • B2 : 1Correct
   • C0.5 : 1
   • D10 : 1
   Why this answer?

   Henderson-Hasselbalch: pH = pKa + log([A−]/[HA]). Rearranging: log([A−]/[HA]) = 5.06 − 4.76 = 0.30. [A−]/[HA] = 10^0.30 ≈ 2. The ratio of acetate to acetic acid is approximately 2:1. This is a standard MCAT buffer question. (Illustrative.)

2. 2

   A galvanic cell has E°cell = +0.46 V at 25 °C. Which statement is correct?

   • AΔG° is positive, and the reaction is non-spontaneous
   • BΔG° is negative, and the reaction is spontaneousCorrect
   • CThe cathode undergoes oxidation
   • DThe anode has the higher reduction potential
   Why this answer?

   ΔG° = −nFE°cell. A positive E°cell gives a negative ΔG°, confirming spontaneity. In a galvanic cell, the cathode undergoes reduction (not oxidation), and the anode has the lower (more negative) standard reduction potential.

3. 3

   For a reaction, ΔH = −40 kJ/mol and ΔS = −100 J/(mol·K). At what temperature does the reaction shift from spontaneous to non-spontaneous?

   • A400 KCorrect
   • B0.4 K
   • C300 K
   • DThe reaction is always non-spontaneous
   Why this answer?

   ΔG = ΔH − TΔS = 0 at the crossover temperature: T = ΔH/ΔS = −40,000 J/mol ÷ (−100 J/mol·K) = 400 K. Below 400 K, ΔG < 0 (spontaneous); above 400 K, ΔG > 0 (non-spontaneous). The negative ΔS means entropy opposes the reaction at high temperatures.