Purpose:

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:  NH4Cl (ammonium chloride), NaCl (sodium chloride or salt), and sand.  Two of the components:  ammonium chloride and sodium chloride are white powders and would be difficult to separate using say a pair of tweezers and a magnifying lens.  However, the 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 chloride, you will take advantage of the fact that it sublimes at a fairly low temperature.  You will heat the entire mixture and the ammonium chloride 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.

Procedure:

Constructing an aluminum weigh boat:

  1. You will want to carry out the ammonium chloride sublimation in an aluminum boat.  But we are VolState, so you will have to make your own.
  2. Obtain a pre-cut piece of heavy gauge aluminum foil.
  3. Use  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 chloride:
  1. Accurately weigh your aluminum boat to the nearest 0.001 g.  (Note:  ALWAYS report every digit from the balance).
  2. On 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).
  3. Accurately weigh (0.001 g) about 5 grams (anywhere from 4-6 grams is OK) 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).
  4. Place your boat and mixture on your hotplate and crank the temperature up to about 75%.  Adjust the snorkel hood so it is about a foot above the hot plate.  This will remove any toxic vapors as the ammonium chloride sublimes.
  5. It may take a couple of minutes for the sublimation to begin.  Make sure the bottom of your boat is flat on the hotplate to ensure efficient heating.
  6. After the ammonium chloride is gone (no more white smoke), continue heating for 1-2 minutes to make sure.
  7. Use your crucible tongs to remove the aluminum boat and allow it to cool to room temperature.
  8. Weight the boat to the nearest 0.001 g.
  9. If you subtract this weight from the weight in Step #2, you will have the mass of ammonium chloride in your mixture.  Q.E.D.
  10. Be sure to turn your hotplate off and let it cool.
Determining the Amount of sand:
  1. Now your boat contains salt and sand.  You will separate them by dissolving the salt with water, leaving the sand behind.
  2. Carefully add about 10 mL of distilled water to your 'boat' and use a disposable pipette to stir it around.
  3. 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!)
  4. Add another 10 mL of distilled water and rinse the boat again to make sure all of the salt has been removed.
  5. Repeat Steps #2 through #4 as necessary until all of the salt has been removed.
  6. Remove as much as the water as you can with the disposable pipette.
  7. Now heat your boat on your hotplate (~50% power) 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 and will have to go back to Step #2).
  8. 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!)
  9. 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 minutes.
  10. Let your boat cool to room temperature and weigh it.
  11. 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.
  12. Now the mass of sand is the mass from Step #1 minus the mass in Step #10.  Q.E.D.
  13. Be sure to turn your hotplate off and let it cool.
Determining the Amount of Sodium Chloride:
  1. There are a number of chemical means to independently determine the amount of sodium chloride in your mixture.  However, due to safety and time constraints, you will determine the amount of sodium chloride by difference.
  2. From the above procedures, you know the weight of the original mixture.  You also know the weight of the ammonium chloride in the mixture, and the weight of the sand in the mixture.  Therefore, the amount of sodium chloride is simply the weight of the mixture minus the weight of the ammonium chloride and the weight of the sand.
  3. Note:  if you made mistakes in the amount of ammonium chloride, or the sand, this will cause an error in your amount of salt.
Calculation of Results:

OK, let's work through an EXAMPLE (your numbers will be different!!) assuming that you obtained the following data:

[A]:  Mass of aluminum 'boat':
  5.000 g 
[B]:  Mass of 'boat' + mixture:
6.500 g
[C]:  Mass of 'boat' + mixture after heating to remove ammonium chloride:
6.100 g
[D]:  Mass of 'boat' + mixture after removing salt, 1st heating:
5.310 g
[E]:  Mass of 'boat' + mixture after removing salt, 2nd heating:
5.300 g

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 chloride:

mass of ammonium chloride  =   [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 chloride and the mass of the salt from the original mass of the the mixture:

mass of salt  =  original mass of mixture  -  mass of ammonium chloride  -  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 percent::

% ammonium chloride  =  100  *  (grams of ammonium chloride  /  grams of original mixture)  =  100  *  (0.400 g  /  1.500 g)  =  26.7%

% 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 9/13/13 by C.R. Snelling)