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Colligative Properties Pre-Lab

Colligative Properties Pre-Lab. Units of Concentration Molarity = Volume dependent Temperature dependent; can’t use when T is changing Molality = Freezing Point Depression ( D T f ) Dissolved solutes lower the freezing point of liquids

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Colligative Properties Pre-Lab

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  1. Colligative Properties Pre-Lab • Units of Concentration • Molarity = • Volume dependent • Temperature dependent; can’t use when T is changing • Molality = • Freezing Point Depression (DTf) • Dissolved solutes lower the freezing point of liquids • Solute molecules block solvent from getting to the crystal to freeze • The Equilibrium is shifted towards the liquid

  2. Colligative Properties = depend only on the amount of solute, not the identity of the solute • Freezing Point Depression • Boiling Point Elevation • Osmotic Pressure • Freezing Point Depression Equation • kf = molal freezing point constant (different for each solvent) • m = molality of the solution Pure Solvent (constant Tf) DTf Temp Solution (Tf falls as concentration increases) Time in ice bath

  3. 3. Data Collection and Processing • (Temp, time) data collected every 15 seconds (6 different runs) i. Take your first point at room temp (before in bath) at time = 0 s ii. Pure cyclohexane freezing point iii. Cyclohexane + 1/5 of your solute sample (~0.16g) iv. Cyclohexane + 2/5 of you solute sample (~0.32g) v. Etc… for a total of 6 freezing point runs (0.48g, 0.64g, 0.80g) • Plot T vs. time • Use Excel to get 2 straight lines, which intersect at Tf • DTf = Tf(pure) – Tf(solution) [6 different plots; 5 different DTf]

  4. 4. Finding the Molecular Weight of the Unknown Force to 0

  5. Units • Hints on doing the experiment • First thing: make your ice bath as cold as possible by adding salt (-6 oC) This will make each run go faster, saving you much time • Stirring during the freezing process is very important—demo

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