Download Unit 9, Section 2B - % Comp and Colligative Prop

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)