Chemistry Reference
In-Depth Information
In this particular problem, you don't even need to pay attention to what
solute is used, because the number of moles you have is indicated. The an-
swer would turn out exactly the same if you were told that the solution was
made with 3.0 moles of HCl or 3.0 moles of NaCl. Let's see the solution to
the problem.
Given: Number of moles of solute = 3.0 moles; liters of solution = 2.0 L
Find: Molarity (M)
Formula
Answer
liters of solution
=
3.0 moles
moles of solute
Molarity (M) =
= 1.5 M
2.0 L
Try to visualize what this answer means, and you will have an easier
time with these calculations. You may recall a time when you made a drink
from a powder mix, and you altered the original recipe given on the con-
tainer. Intuitively, you could figure out how to make your lemonade or
iced tea twice as strong or half as strong. Molarity calculations work in
much the same way. When we find that our answer is 1.5 M, it means that
there are 1.5 moles of solute in every liter of solution. If you were to use
twice as much solute, you would double the concentration of the solution,
making it twice as strong, because 6.0 moles/2.0 L = 3.0 M. If you were to
double the volume of the solution without adding more solute, you would
dilute the solution to half its strength, because 3.0 moles/4.0 L = 0.75 M.
Slightly more difficult examples of molarity problems involve an extra
calculation. Remember: You need to know the number of moles of solute
in order to calculate the molarity of a solution. Sometimes you will start
with the mass of the solute and you will need to determine the number of
moles of solute that you are starting with by using a formula that you stud-
ied in Chapter 7:
mass of the sample
molar mass of the substance
Number of moles of a substance (n) =
Let's try a molarity problem that involves two calculations.
Example 2
Determine the molarity of a 2.5 L solution made with 200.0 g of
calcium chloride (CaCl
2
).
Can you see how Example 2 differs from Example 1? In order to solve
for molarity, we need to know how many moles of solute that we have. In
Example 2, we're given the mass of the solute, instead of the number of
moles. To solve this problem we need to:
