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)