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Unit 9: Solution Chemistry Section 2: Percent Composition and Colligative Properties Percent Composition • Used by consumers to determine the concentration of a solution – Chemists use molarity! • There are formulas used to calculate % composition – Consumer products don’t indicate which formula they are using – Each manufacturer chooses their own method that they feel is most suitable for their product Possible issues????? Percent Solutions • Mass/mass solutions: – Ex: Calculate the percent solution formed by mixing 5 grams of solute with 95 grams of solvent. solution Percent Solutions • Mass/Volume solutions: – Ex: Calculate the percent solution formed by mixing 2 grams of solute with enough water to make 50 mL of solution. Percent Solutions • Volume/Volume solutions: – Ex: Calculate the percent solution formed by mixing 5 mL of alcohol with enough water to make 100 mL of solution. Other Concentration Terms • Parts per million (ppm): • Parts per billion (ppb): • “Parts” = basic unit • These terms are usually reserved for describing relatively small amounts, such as levels of air and water pollutants Colligative Properties • Properties of solutions that depend on the number of solute molecules in a given volume of solvent Vapor Pressure • Caused by the evaporation of molecules at the surface of a liquid – The escaping molecules exert an upward pressure as they leave the liquid • Solutions have less vapor pressure because the solute molecules mixed in reduce the number of solvent molecules on the surface. Boiling Point • The temperature at which the vapor pressure escaping from a liquid exceeds atmospheric pressure pushing down on the surface of the liquid – The boiling point of a liquid increases when molecules of a solute are added – surface blocked by molecules of solute Freezing Point • The temperature at which molecules of a substance no longer slide past each other; becomes a solid. – When a solute is dissolved, the freezing point is lowered (a lower temperature must be reached to freeze the solution)