Untitled - Kelly Walsh High School

180 CHEMISTRY FOR THE UTTERLY CONFUSED
falling between solutions and suspensions. These are colloids and have particles
in the 1 to 10 3 nm diameter range. Smoke, fog, milk, mayonnaise, and latex
paint are all examples of colloids.
Many times, it is difficult to distinguish a colloid from a true solution. The most
common way is by shining a light through the mixture. A light shone through a
true solution is invisible, but a light shown through a colloid is visible due to the
reflection of the light off the larger colloid particles. This is the Tyndall effect.
12-5 Utterly Confused About
Colligative Properties
Let’s examine a few examples of how to approach colligative property problems.
We will begin with a Raoult’s law example.
What is the vapor pressure of a solution made by mixing 80.0 g of chloroform,
CHCl 3, in 800.0 g of carbon tetrachloride, CCl 4? The vapor pressure of chloroform
is 197 torr, and the vapor pressure of carbon tetrachloride is 114 torr (all
vapor pressures are determined at 25°C).
Raoult’s law requires the mole fraction of each volatile material. Whenever the
mole fraction is not present, we will need to calculate the value from the number
of moles. In this example, we must begin by calculating the moles of each
constituent of the solution.
Moles chloroform (80.0 g CHCl 3) (1 mol CHCl 3/119.378 g CHCl 3)
0.6701402 mol CHCl 3 (unrounded)
Moles carbon tetrachloride (800.0 g CCl4) (1 mol CCl4/153.823 g CCl4) 5.2007827 mol CCl4 (unrounded)
The mole fraction of chloroform (solute) is:
X solute
(0.6701402) mol CHCl3 0.1141456 (unrounded)
(0.6701402 5.2007827) mol
It is possible to calculate the mole fraction of carbon tetrachloride (solvent) in
a similar manner. However, simply subtracting the mole fraction of chloroform
from 1 will give the same value.
X solvent 1 0.1141456 0.8858544 (unrounded)