The purpose of today's lab is to
completely separate a mixture into its components using only physical
means. Your mixture will contain three components: (NH4)2CO3
(ammonium carbonate), NaCl (sodium chloride or salt), and colored
Two of the components: ammonium carbonate and sodium chloride are
white powders and would be difficult to separate using say a pair of
tweezers and a magnifying lens. However, the other component,
colored sand, could probably be separated this way (but that is a story
for another day).
You will be using a combination of physical properties to
quantitatively separate these components. To remove the ammonium
carbonate, you will take advantage of the fact that it sublimes at a
very low temperature. You will heat the entire mixture and the
ammonium carbonate will turn into a gas leaving the salt and sand
behind. Next, you will use the fact that salt is very water
soluble, but the sand is not. You will add water to the salt
and sand that remains and this will dissolve the salt leaving the sand behind. You will then 'decant' the water leaving the sand behind. After you dry the sand, you will determine its mass.
Constructing an aluminum weigh boat:
- You will want to carry out the ammonium carbonate
sublimation in an aluminum boat. But we are VolState, so you will
have to make your own.
- Obtain a pre-cut piece of heavy gauge aluminum foil.
your 250 mL beaker as a mold and fold the aluminum
foil around the base until you obtain something like the figure
below (use a pair of scissors to trim your boat). Make sure the
bottom is flat and that the sides are as smooth as possible to make it
water tight. (Note: the photo below
shows two types of commercially (i.e., more money than we want to
spend) available weigh boats....I don't expect yours will look quite
this finished, but they do give you the idea of what you are after).
Determining the Amount of Ammonium Carbonate:
- Accurately weigh your aluminum boat to the nearest 0.001 g. (Note: ALWAYS report every digit from the balance).
the back bench, you will find four containers labeled A, B, C, and
D. Pick one and shake it vigorously for 10-20 seconds to make
sure it is evenly mixed (homogeneous).
- Accurately weigh (0.001 g) about 5 grams on one of your mixture into your aluminum boat and reweigh. (Note: ALWAYS remove the boat from the balance before adding or removing mixture; then put the boat back onto the balance).
your boat and mixture on your hotplate and crank the temperature up to
about half way. Adjust the snorkel hood so it is about a foot
above the hot plate. This will remove any toxic vapors as the
ammonium carbonate sublimes.
may or may not see any vapor as the ammonium carbonate sublimes, but
keep heating for about five to ten minutes. However, if you
start to see any tan color forming in the boat, then stop heating and
remove the aluminum boat and let it cool.
- Use your crucible tongs to remove the aluminum boat and allow it to cool to room temperature.
- Weight the boat to the nearest 0.001 g.
- If you subtract this weight from the weight is Step #2, you will
have the mass of ammonium carbonate in your mixture. Q.E.D.
Determining the Amount of sand:
- Now your boat contains salt and colored sand. You
will separate them by dissolving the salt with water, leaving the sand behind.
- Carefully add about 5 mL of distilled water to your 'boat' and use a disposable pipette to stir it around.
- Use the disposable pipette to remove as much as the water as you can. (Note: be very careful that you do not remove any of the sand!)
- Add another 5 mL of distilled water and rinse the boat again to make sure all of the salt has been removed.
- Repeat Steps #2 through #4 as necessary until all of the salt has been removed.
- Remove as much as the water as you can with the disposable pipette.
- Now heat your boat on your hotplate for about 15 minutes to remove any traces of water. (Note: if you see any white spots appear, that means you did not remove all of the salt).
- Let your boat cool to room temperature and weigh it. (Note: it is VERY important that an object is at room temperature before you weigh it!)
- At this point, we are not sure that all of the water has been
removed, so we will heat it again on the hotplate for another 10
- Let your boat cool to room temperature and weigh it.
- If the weight from Step #10 and Step #8 are within 0.01g of each
other, you are done heating. If not, you will need to heat your
boat a third time.
- Now the mass of sand is the mass from Step #1 minus the mass in Step #10. Q.E.D.
- Believe it or not, the colored sand is fairly expensive, so your instructor will be collecting it so we can use it again.
Determining the Amount of Sodium Chloride:
- There are a number of chemical means to independently determine
of sodium chloride in your mixture. However, due to safety and time
you will determine the amount of sodium chloride by difference.
- From the above procedures, you know the weight of the original mixture.
You also know the weight of the ammonium carbonate in the mixture, and the weight of
the sand in the mixture. Therefore, the amount of sodium chloride is
the weight of the mixture minus the weight of the ammonium carbonate and the weight
- Note: if you made mistakes in the amount of ammonium
carbonate, or the sand, this will cause an error in your amount of
Calculation of Results (Do
Not include this in your Pre-lab):
OK, let's work through an EXAMPLE (your
will be different!!) assuming that you obtained the following data:
|[A]: Mass of aluminum 'boat':
| 5.000 g
|[B]: Mass of 'boat' + mixture:
|[C]: Mass of 'boat' + mixture after heating to remove ammonium carbonate:
|[D]: Mass of 'boat' + mixture after removing salt, 1st heating:
|[E]: Mass of 'boat' + mixture after removing salt, 2nd heating:
So, what is the % of each component?
First we need to calculate the mass of the original mixture:
mass of the mixture = [B] - [A]
= 6.500 g - 5.000 g = 1.500 g
Now we can calculate the mass of the ammonium carbonate:
mass of ammonium carbonate = [B] -
[C] = 6.500 g - 6.100 g = 0.400 g
Likewise, we can calculate the mass of the sand:
mass of sand = [E] - [A] = 5.300 g - 5.000 g = 0.300 g
We can then determine the mass of the salt, by subtracting the mass
of the ammonium carbonate and the mass of the salt from the original
mass of the the mixture:
mass of salt = original mass of mixture - mass of ammonium carbonate - mass of sand
mass of salt = 1.500 g - 0.400 g - 0.300 g = 0.800 g
Now that we have the mass of each component, we simply divide each
by the total mass of the mixture and multiple by 100 to get the
% ammonium carbonate = 100 * (grams of
ammonium carbonate / grams of original mixture)
= 100 * (0.400 g / 1.500 g) =
% sand = 100 * (grams of sand
/ grams of original mixture) = 100 * (0.300
g / 1.500 g) = 20.0%
% salt = 100 * (grams of salt / grams
of original mixture) = 100 * (0.800 g
/ 1.500 g) = 53.3%
(Updated 4/21/13 by C.R. Snelling)