Define osmosis and osmotic pressure. Why is osmotic pressure preferred for finding the molar mass of proteins? Explain reverse osmosis and give one use.

1. 1 · Define osmosis. The flow of solvent through a semipermeable membrane from the pure solvent (or dilute solution) into the more concentrated solution.
2. 2 · Define osmotic pressure. The extra pressure applied on the solution to just stop osmosis. For dilute solutions, π = CRT = (n₂ ÷ V) RT.
3. 3 · Draw the set-up. Beaker of water; inverted thistle funnel with an SPM over its mouth, holding sugar solution; level rising in the stem; a piston showing π. Label solvent, solution, SPM.
4. 4 · Give the molar mass formula. M₂ = (w₂RT) ÷ (πV).
5. 5 · Explain why it is preferred.
   • Measured at room temperature, so proteins are not damaged by heat
   • π is large even for very dilute solutions, while ΔTb and ΔTf would be too small
   • Proteins and polymers dissolve only a little, so their solutions are very dilute
6. 6 · Explain reverse osmosis. A pressure greater than π on the solution side forces pure water out through the membrane (cellulose acetate). It is used for desalination of sea water and in home RO purifiers.
7. 7 · Close in one line. So osmosis moves water into the stronger solution, and reverse osmosis drives it back out.

Scoring tip: In the definitions, write 'semipermeable membrane' and 'just stop'. These two phrases carry the marks.