Atlas MCAT Course
Equations & How to Use Them → Equations: Equilibrium, Acids & Solutions
Equations: Equilibrium, Acids & Solutions
K, pH, buffers, and colligative properties.
01The idea
A reaction can go forward and backward; pH or buffering matters. Or solutes change boiling, freezing, vapor pressure, or water movement.
THREAD · Gradients and flow
02Build the model
AEquilibrium constant: how far a reaction lies to products
Use when: a reversible reaction is at equilibrium; “equilibrium mixture” or Keq is given/asked; you compare product-favored vs reactant-favored systems.
Passage cues: “At equilibrium”, “Keq”, “Product-favored / reactant-favored”.
Don’t use it for: not at equilibrium (use Q).
BReaction quotient: where you are right now
Use when: you’re given current concentrations and asked the direction of shift.
CStandard free energy vs. equilibrium: linking thermodynamics to K
Use when: you’re given K and asked spontaneity under standard conditions; you’re given ΔG° and asked for K (rare math).
DpH and pOH: how acidic or basic a solution is
Use when: any problem gives [H⁺], [OH⁻], pH, or pOH; you’re converting between acidity measures.
EAcid dissociation constant: acid strength
Use when: comparing acid strength; weak acid equilibria appear.
FWater autoionization: the baseline for pH math
Use when: converting [H⁺] ↔ [OH⁻]; neutrality vs acidity in water.
GHenderson–Hasselbalch: buffers: how pH responds to added acid or base
Use when: buffer solutions appear; you’re given a conjugate base/acid ratio; “buffer capacity” or “resists pH change”.
HMolarity and osmotic pressure: solutions and water movement
Use when: osmosis, tonicity, IV fluids; semipermeable membranes.
IRaoult’s law: vapor pressure lowering
Use when: vapor pressure lowering / colligative property setup.
03See the connections
The lesson names these links itself. Read the map left to right.
Equilibrium
- K vs Q
- ΔG° = −RT ln K
Acids and bases
- pH, pOH, Kw
- Ka, pKa
- Henderson–Hasselbalch
Solutions
- Π = iMRT
- Raoult’s law
Next: waves, optics, and modern physics.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
For a reaction, Q is greater than K. Which way does it shift?
Think first.
A solution’s pH drops from 7 to 5. How has [H⁺] changed?
Think first.
A buffer has equal concentrations of weak acid and conjugate base. What is its pH?
Think first.
Which solution has the higher osmotic pressure: 0.1 M glucose or 0.1 M NaCl?
05MCAT lens
MCAT shortcuts
Big K → products dominate
Q < K → right; Q > K → left
Each 1 pH unit = 10× [H⁺]
When [A⁻] = [HA], pH = pKa
More particles → higher osmotic pressure
More solute → lower vapor pressure
06Common traps
Trap 1: Using K when the system isn’t at equilibrium
Misconception
K describes the end point.
Correct model
Use Q for current conditions.
Trap 2: A small pH change is a small acidity change
Misconception
pH is logarithmic.
Correct model
Each unit is 10×.
Trap 3: Osmotic pressure depends on molarity alone
Misconception
Particle count matters.
Correct model
Π = iMRT.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
When to use K vs Q
Check against 02 · A and B.
How ΔG° relates to K
Check against 02 · C.
How buffers set pH
Check against 02 · G.
Why particle count matters for solutions
Check against 02 · H and I.
If something feels unclear, the missing idea is usually thisCompare where a system is with where it settles; count particles for solutions.
08Remember
REMEMBER · if you keep only a few things
- K: where it settles. Q: where it is now.
- ΔG° = −RT ln K.
- pH = −log[H⁺]; pH + pOH = 14.
- Kw = 10⁻¹⁴; neutral [H⁺] = 10⁻⁷.
- pH = pKa + log([A⁻]/[HA]).
- Π = iMRT; Raoult: more solute, lower vapor pressure.
09Forward hook
Equilibrium and solutions close out the chemistry equations.
Which equation applies when energy propagates through space or a medium?
That leads to waves, optics, and modern physics.
Next lessonEquations: Waves, Optics & Modern Physics→