Another similar way of expressing the concentration of a solution is to express it in weight
percent (or mass percent, if you prefer).
|Weight percent =
weight of solute
weight of solution
You've done this kind of calculation earlier - last term in the lesson on composition.
You may already be prepared to answer the questions in exercise 2 in your workbook. If so, do
that and check your answers at the bottom of the page. If not, read on.
|As an example, let's consider a 12% by weight sodium chloride
solution. Such a solution would have 12 grams of sodium chloride for every 100 grams of
solution. To make such a solution, you could weigh out 12 grams of sodium chloride, and
then add 88 grams of water, so that the total mass for the solution is 100 grams. Since
mass (unlike volume) is conserved, the masses of the components of the solution, the
solute and the solvent, will add up to the total mass of the solution.
||12 % NaCl solution =
|| 12 g NaCl
100 g solution
12 g NaCl
(12 g NaCl + 88 g water)
|= 12% NaCl solution
To calculate the mass percent or weight percent of a solution, you must divide the mass
of the solute by the mass of the solution (both the solute and the solvent together) and
then multiply by 100 to change it into percent.
Examples (Ex. 1)
Your workbook has some examples of calculations involving weight percent in example 1.
An explanation of those examples is given here.
|Example 1-a asks, "What is the weight percent of glucose in a
solution made by dissolving 4.6 g of glucose in 145.2 g of water?" The way that I
recommend you go about doing this is to look at what you need to find, look at what you
are given, and determine what the relationship is. Let's start with what you are trying to
find, the weight percent of glucose in the solution. What do we need in order to calculate
that? We need to divide the weight of glucose by the weight of the solution. We have the
weight of glucose, that is 4.6 g. What about the weight of the solution? That is not
given, but we can figure it out by adding together the weight of glucose and water to get
149.8 g. Now we can calculate the weight percent of glucose as shown to get 3.1%.
What is the weight percent of glucose in a solution made by dissolving 4.6 g of glucose in
145.2 g of water?
To get weight percent we need the weight of the solute and the total weight of the
|Determine total weight of solution:
+ 145.2 g
Weight % glucose =
4.6 g glucose x 100 = 3.1% glucose
149.8 g solution
|The next question is a little bit different. You are asked how you would
prepare 400 g of a 2.50% solution of salt. You are given 400. g of solution (that is total)
and you know that 2.50% of that is going to be salt. You need to find out how much salt
you need and how much water you need. You can simply multiply 400. g by 2.50% to find out
how much salt there is (shown in the top line), or you can set up the calculation shown on
the next line. Either way, you find that you need 10.0 g of salt. Since you need a total
mass of 400. g and 10. g of that is salt, the remaining 390. g would have to be water. So, to
prepare this solution you would have to mix 10.0 g of salt with 390. g of water.
How would you prepare 400. g of a 2.50% solution of sodium chloride?
We need to find out how much salt is needed and how much water is needed.
|Determine weight of salt:
|400. g x 2.50% salt = 10.0 g salt
|400. g solution x
2.50 g salt
100 g solution
|= 10.0 g salt
|Determine weight of water:
- 10. g
Dissolve 10.0 g salt in 390. g water.
If you have any questions about these calculations be sure to stop and go over them
again or work with the instructor if you need to, so that you can get squared away on how
to work these kinds of problems.
Practice (Ex. 2)
Take time now to answer the following questions (also given in exercise 2 in your
workbook). The third question, you will note, has an extra twist to it. Take some time to
do these now, get some help if you need it and then check your answers below.
|What is the weight percent of ethanol in a solution made by dissolving 5.3 g of ethanol
in 85.0 g of water?|
|How would you make 250. g of a 7.5% solution of glucose in water?|
|A sample of a solution weighing 850.0 g is known to contain .223 moles of potassium
chloride. What is the weight percent of potassium chloride in the solution?|
Answers (Ex. 2)
The answer to "a" is 5.9% ethanol. If you got
6.2% probably what you did was to not add together the mass of the ethanol and the water
to get the total mass of the solution.
Question "b" brings up a couple points. One is that the question asks
"how would you prepare?" So the answer is not just 18.75g of glucose. Since the
question asks you how to do something, you need to answer with how you would do it. You
would dissolve 18.75 g of glucose in 231.25 g of water. The
other point this brings up is a matter of significant digits. According to the number of
digits given in the question it would be very legitimate to say that you would dissolve 19
g of glucose in 231 g of water. That would be a very appropriate thing to do. However, if
you are trying to prepare a solution, try to prepare it as well as you can. Get as close
to the 7.5% as you can. If you were to round this value to 19g and then dissolved it in
231 g of water, what would actually prepare would be 7.6% solution. In terms of
significant digits, that is not a problem, you just have a slight variation in the last
digit and that fits. But if you want to prepare a 7.5% solution instead of a 7.6%
solution, then you have to be more careful than just two significant digits.
The answer to "c" is 1.96% potassium chloride.
Depending on how you went about doing your calculations and rounding off you may have
gotten 1.95% instead of 1.96%. Don't worry too much about that. One more point about the
last question here is that in order to do it you needed to change from moles of KCl to
grams of KCl. This is just the opposite of what you will need to do when calculating
molarity a little later in this lesson.
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