Concentration & Solubility
World B — Acids, Bases & Solutions
19.A — MEASURING CONCENTRATION
R19.01 Qualitative Concentration
- Dilute: Very little solute relative to solvent.
- Concentrated: Large amount of solute.
- A purely comparative description.
1L
R19.02 Molarity (M)
- Moles of solute / Liters of Solution.
- Volume-dependent.
- Changes with temperature (liquids expand).
1 kg
MASS
R19.03 Molality (m)
- Moles of solute / Kilograms of Solvent.
- Mass-dependent.
- Independent of temperature changes.
X₁
X₂
R19.04 Mole Fraction (X)
- Moles of component / Total moles in mixture.
- Unitless ratio.
- Sum of all mole fractions = 1.
R19.05 Mass Percentage
- (Mass of Solute / Total Mass of Solution) × 100.
- Often written as % w/w.
- Common for solid-in-liquid solutions.
R19.06 Parts Per Million (ppm)
- For extremely trace quantities.
- Mass of solute / Mass of solution × 10⁶.
- Used for pollutants in air/water.
19.B — SOLUBILITY DYNAMICS
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R19.07 Saturation Point
- Maximum solute dissolved at a specific temperature.
- Dynamic equilibrium: Dissolving rate = Crystallizing rate.
- Any extra solute falls to the bottom.
R19.08 Supersaturation
- Solution holds more solute than saturation limit.
- Created by heating, dissolving, and slow cooling.
- Highly unstable; seed crystal causes rapid precipitation.
R19.09 Temp Effect (Solids)
- For most solids in liquids, solubility increases with heat.
- Heat provides kinetic energy to break solute bonds.
R19.10 Temp Effect (Gases)
- For gases in liquids, solubility DECREASES with heat.
- Increased kinetic energy allows gas molecules to escape.
- Reason soda goes flat when warm.
R19.11 Henry's Law
- Gas solubility is directly proportional to partial pressure above liquid.
- Pushing piston down (Pressure↑) forces gas into solution (Solubility↑).
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R19.12 The Dilution Law
- $M_1V_1 = M_2V_2$.
- Moles of solute remain constant before and after dilution.
- Only volume changes, lowering concentration.