Chemistry Reference
In-Depth Information
As you can see, before you can calculate the molality of the solution,
you will need to determine the number of moles of solute that you are
starting with. You can do this by dividing the mass of the solute (CaCl
2
) by
its molar mass. You may recall from Example 2 that the molar mass of
CaCl
2
is 111.1 g/mole. Once you know the number of moles of solute that
you have, you will be able to divide that value by the mass of the solute (1.5
kg of water) in order to determine the molality. Let's see what this would
look like.
Given: mass of solute = 120 g; kilograms of solvent = 1.5 kg
Find:
A. number of moles of solute
B. molality of solution
mass of sample
molar mass
120.0 g
111.1 g
A.
Number of moles of solute =
=
= 1.08 moles of CaCl
2
moles of solute
kilograms of solvent
1.08 moles
1.5 kg
B.
Molality =
=
= 0.72 m or 0.72 molal
Working with a different unknown shouldn't really be any harder than
the problem from Example 5. Let's try a problem where the mass of the
solute is the unknown. Take a look at the problem and try to figure out the
given information and what you need to find. Include the values that you
will need to find in the order that you will need to find them.
Example 6
How many grams of strontium chloride (SrCl
2
) must you add to
2.50 kg of water to produce a solution with a molality of 0.500 m?
Given:
Find:
You should have made note of the fact that you have been given the
number of kilograms of the solvent (2.50 kg) and the molality (0.500 m) of
the solution. You want to find the mass of the solute, but you need to find
two other pieces of information before you can do that. First, you want to
use the molality formula to determine the number of moles of solute that
you need to produce a solution of the required concentration. Next, you
want to determine the molar mass of the solute, so that you can use that
information to change number of moles to mass.
