Study TipsMay 19, 20267 min read
MDCAT Chemical Equilibrium 2026 — Le Chatelier, K<sub>c</sub>/K<sub>p</sub>, Buffers
Premeth Team
Meth Experts
Chemical Equilibrium yields 3–4 of the 45 MDCAT Chemistry MCQs every paper. Le Chatelier's principle is the highest-frequency single topic. Master the Kc ↔ Kp conversion and buffer calculations.
- Estimated MCQs — 3–4 of 45 Chemistry
- Highest-yield subtopic — Le Chatelier's principle predictions
- Trap concept — catalysts do NOT shift equilibrium
Source: PMDC MDCAT 2026 syllabus.
Le Chatelier's Principle
If a system at equilibrium is disturbed, the equilibrium shifts to counteract the disturbance.
- Add reactant → shifts forward (right)
- Add product → shifts backward (left)
- Increase pressure → shifts toward fewer gas moles
- Increase temperature → shifts away from heat (endothermic forward = shifts right; exothermic forward = shifts left)
- Catalyst → speeds up both directions equally; does NOT shift equilibrium
Kc and Kp
- Kc — equilibrium constant in terms of concentration. Kc = [products]n/[reactants]m
- Kp — in terms of partial pressure of gases.
- Kp = Kc(RT)Δn where Δn = gas moles products − gas moles reactants
- K > 1 — products favoured; K < 1 — reactants favoured.
Buffers (acid-base equilibrium)
- Acidic buffer — weak acid + its salt (e.g., CH3COOH + CH3COONa).
- Basic buffer — weak base + its salt (e.g., NH4OH + NH4Cl).
- Henderson-Hasselbalch — pH = pKa + log([salt]/[acid]).
- Blood buffer — H2CO3 / HCO3−, maintains pH ~7.4.
FAQ
Q: Does volume affect equilibrium?
Yes, indirectly — via concentration change. Decreasing volume increases concentration; shifts toward fewer moles of gas.
Q: Are solids/liquids in K expression?
No — only aqueous solutions and gases.
Pair with our chapter page and drill past-paper MCQs.
Related Tags
#MDCAT#Chemistry#Equilibrium#Le Chatelier
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