How can I prepare solutions with specific
solute concentrations when I have separate solutes and solvent?
The first method that we will examine for preparing
solutions is used when you are starting with separate solutes and solvent. In
other words, there is no existing stock solution. In this case, you dissolve
the solutes (usually solid) in the solvent (usually dH20.) To make
a solution in this way, you must know how much of each solute to use and how
much solvent to use.
As mentioned previously, a 1 molar (1 M) solution contains
1 mole of solute dissolved in 1 liter of total solution. Obviously, we cannot
measure moles directly because we have no way to count atoms or molecules.
However, the formula weight of a substance tells us how much one mole of
the substance weighs. Therefore, we can use the formula weight to measure
moles indirectly.
Potassium permanganate
Graduated cylinder with water
YOUR TURN
You want to prepare one liter of a 2 M
potassium permanganate (KMnO4) solution.
How many moles of potassium permanganate should you dissolve in one liter
of solution?
The formula weight of KMnO4 is 158.04 g/mole.
How much potassium permanganate should you weigh out in order to prepare one
liter of the following KMnO4 solutions?
Suppose you do not need an entire liter of solution. How
much potassium permanganate should you weigh out in order to prepare the
following amounts of a 0.75 M KMnO4 solution?
Once you have weighed out the required amount of solute, place it in
an appropriate volumetric measuring device (such as a graduated cylinder) and
add enough solvent until the total volume of the solution reaches the desired
amount. Notice that if you want to make 80 mL of solution, you must add less
than 80 mL of solvent to the solute because the solute occupies some volume. If
you added a full 80 mL of solvent to the solute, the total volume of the
solution would be greater than 80 mL, and your solution would be too dilute.
General Procedure for Preparing a Solution by Dissolving a Solid Solute in
the Solvent:
Determine the amount of solute you need,
and weigh it out with an electronic balance.
Place the solute in an appropriate
volumetric measuring device. For example, if you need 80 mL of
solution, a 100 mL graduated cylinder would be an appropriate choice.
Carefully add water to your measuring
device until the bottom of the meniscus reaches the level of the final
volume of your solution.
Seal the top of the measuring device so
no solution can leak out. In the case of a graduated cylinder, you can
seal it by stretching and tightly wrapping a piece of Parafilm around
the top and holding it securely in place with your fingers or the palm
of your hand (make sure you are wearing protective gloves.)
Gently invert the measuring device
several times until all of the solute is dissolved.
YOUR TURN
Your Turn
A student wants to make 80 mL of a 1.0 M solution of
sucrose (formula weight 342.3 g/mole). How much sucrose should he weigh
out to prepare this solution?
The student weighs out the correct amount of sucrose,
places it in a beaker, measure 80 mL of dH2O with a graduated cylinder,
adds the dH2O to the beaker, and then mixes the solution with a stir bar
until all of the sucrose is dissolved. What did he do wrong?
(Notice how all the
units except for grams cancelled out of this equation!)