Chapter 9 — Solutions: Long Answer Questions
Factors Affecting Solubility
Mass Percentage (w/w %)
Definition: Mass of solute per 100 g of solution.
Example: 25 g of NaCl dissolved in 225 g of water.
- Mass of solution = 25 + 225 = 250 g
- Mass % NaCl = (25/250) × 100 = 10%
Molarity (M)
Definition: Number of moles of solute per litre of solution.
Unit: mol/L or mol dm⁻³
Example: 5.85 g of NaCl (molar mass = 58.5 g/mol) dissolved to make 500 mL of solution.
- n(NaCl) = 5.85/58.5 = 0.1 mol
- V = 500 mL = 0.5 L
- M = 0.1/0.5 = 0.2 M
Molality (m)
Definition: Number of moles of solute per kilogram of solvent.
Unit: mol/kg
Advantage over molarity: Molality is independent of temperature (uses mass, not volume). Molarity changes slightly with temperature.
Example: 10 g of glucose (C₆H₁₂O₆, molar mass = 180 g/mol) dissolved in 500 g of water.
- n(glucose) = 10/180 = 0.0556 mol
- Mass of solvent = 500 g = 0.5 kg
- m = 0.0556/0.5 = 0.111 mol/kg
Osmosis
Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable membrane from a region of lower solute concentration (higher solvent concentration) to a region of higher solute concentration (lower solvent concentration).
Semi-permeable membrane: A membrane that allows solvent molecules to pass through but not solute particles. Examples: cell membranes, egg membrane, parchment paper, RBC membrane, dialysis tubing.
Driving force: The difference in solvent chemical potential (or water potential) across the membrane.
Osmotic Pressure (π)
The minimum pressure that must be applied to the solution to just stop the osmosis of solvent from pure solvent into the solution.
Van't Hoff equation:
Where M = molarity, R = gas constant, T = temperature in K
Higher solute concentration → higher osmotic pressure.
Types of solutions with respect to a biological cell:
- Isotonic: Same concentration as cell contents — cell maintains normal size
- Hypotonic: Less concentrated than cell — water enters cell by osmosis → cell swells → may burst (lysis in animal cells; turgidity in plant cells)
- Hypertonic: More concentrated than cell — water leaves cell by osmosis → cell shrinks (plasmolysis in plants; crenation in animal cells)
Daily Life and Biological Examples of Osmosis
| Example | Explanation |
|---|---|
| Salting vegetables and meat | Salt draws out water by osmosis (food preservation — bacteria dehydrate) |
| Plants absorbing water from soil | Soil water (hypotonic) moves into root cells (hypertonic) by osmosis |
| Wilting of plants | Loss of water (drought/high soil salinity) → cells lose turgidity → wilting |
| Rehydration salts (ORS) | Balanced ionic concentration helps intestine absorb water by osmosis |
| Kidney dialysis | Artificial semi-permeable membrane removes waste from blood |
| Reverse osmosis (RO) water purifiers | High pressure applied to force water through membrane against osmotic gradient — removes dissolved salts |
| Preserving food in jam/jelly | High sugar concentration draws water out of microbes by osmosis — preserves food |
Boiling Point Elevation (ΔTb)
When a non-volatile solute is dissolved in a solvent, the boiling point of the solution is higher than that of the pure solvent.
Reason: The vapour pressure of the solution is lower than pure solvent (Raoult's Law). More heat must be supplied to raise vapour pressure to atmospheric pressure → higher boiling point.
Where Kb = ebullioscopic constant of solvent, m = molality
Practical example: Adding salt to cooking water raises the boiling point → food cooks faster (though the increase is very small — about 0.5°C per mole per kg for water).
Freezing Point Depression (ΔTf)
When a solute is dissolved in a solvent, the freezing point of the solution is lower than that of the pure solvent.
Reason: Solute particles interfere with the formation of the ordered crystal lattice of the solid → lower temperature needed to freeze.
Where Kf = cryoscopic constant of solvent, m = molality
Practical examples:
- Antifreeze in car radiators: Ethylene glycol dissolved in water lowers the freezing point below −40°C → prevents radiator from freezing in winter
- Salting roads in winter: Salt (NaCl) dissolves on wet roads, depressing the freezing point → prevents ice formation
- Sea water: Dissolved salts lower the freezing point to about −1.8°C → ocean freezes at a lower temperature than fresh water
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