LAB – PERCENT COMPOSITION OF A MIXTURE
CONTEXT
In this lab, you have a mixture of NaHCO3, sodium bicarbonate, and Na2CO3, sodium
carbonate. These two salts have different properties that will allow you to ultimately
determine the amount of each chemical in the mixture.
This experiment involves a major practice of chemists: separating substances in a mixture by
taking advantage of properties of the substances that are unique to each one. In this case,
you will rely on the substances’ chemical reactivity upon heating as the property that differs
between them. From antiquity, two very important substances in society have been obtained
from a salt mixture called natron. Natron has been harvested for thousands of years from dry
lake beds. In ancient Egypt, and still today, natron is blended with oil and used as soap.
Natron primarily consists of two substances, sodium carbonate (Na 2CO3) and sodium
bicarbonate (NaHCO3). Each of these substances, when separated, also has important uses.
Sodium carbonate is used in the manufacture of glass, as a water softener for laundry, as an
additive in community swimming pools to raise pH, and as an additive in foods. Sodium
bicarbonate has many uses, ranging from cooking and medical uses, to cleaning, pesticide,
and fire extinguishing uses.
Sodium carbonate melts at 851°C. At temperatures above 250°C it slowly decomposes into
carbon dioxide and sodium oxide. Sodium bicarbonate is much less stable with respect to
heating. When heated to around 100°C, sodium bicarbonate will decompose into sodium
carbonate solid, water vapor, and carbon dioxide. Around 200°C this decomposition is
complete. An interesting fact about sodium bicarbonate is that when it is dissolved in water,
carbon dioxide production begins at room temperature and the decomposition of is
essentially complete if the solution is brought to boiling temperature. The ease with which
sodium bicarbonate loses carbon dioxide on heating is what makes it useful as "baking soda"
and as a component in baking powders; channels opened by escaping carbon dioxide
bubbles give baked goods a lighter and fluffier texture.
You will heat a mixture of solid sodium carbonate and solid sodium bicarbonate over your
kitchen stovetop or a hotplate. (These typically do not reach temperatures higher than
250°C.) Therefore only one of the decompositions mentioned above will occur. You will use
this fact to determine a stoichiometric method to find the amounts of sodium bicarbonate and
sodium carbonate in the original mixture. Finally, calculate the percent composition of the
two chemicals in the original mixture.
MATERIALS:
Chemicals
Vial containing a mixture of sodium
bicarbonate
(NaHCO3) and sodium carbonate
(Na2CO3) in unknown amounts
Student Provided
Stovetop or Hotplate
Equipment
Wire gauze Test tube
Metal scoop Test tube holder
Test tube
Balance
You may use any of the containers in your kit as needed.
SAFETY and DISPOSAL
Do not heat covered test tubes. Hot test tubes should be held with tongs or your own pot
holder. Normal laboratory precautions should be taken, including wearing goggles at all
times. Information about each chemical can be found in the safety section.
PROCEDURE
Read all the instructions and make a data table before you begin. Think ahead to determine
what calculations you will do to determine the percent composition of the mixture. Then work
backwards to make your data table accordingly.
1. Turn on your digital balance. Place an empty
beaker on the balance and press “tare” to zero the
balance.
2. Put a clean, dry test tube in a beaker and record
the mass. You are using the beaker to keep the
test tube from rolling off the balance.
3. Shake up the mixture in its closed container to
break up any pieces that may have settled upon
shipping. If needed, stir the mixture or break it up
with your stirring rod.
4. Take the test tube out of the beaker and use the scoop to transfer about 5 g of the
mixture to the test tube. Transferring of substances should never happen directly over
the balance. Always move the container away from the balance first.
5. Put the test tube in the beaker (still zeroed) and record
the exact mass.
6. Bend the edges of the wire gauze up to make a container
to hold the test tube while heating (as shown to the right).
If you are using a hot plate or oven, use a small oven or
stove safe dish to hold the test tube. The beakers and
flasks in this kit are plastic. Do not put plastic
beakers or flasks over any heat source! If you use a flame source (like a gas stove,
Bunsen burner, alcohol burner, etc.) make sure that the flame remains at the bottom of
the test tube. It should not be so large as to engulf the entire test tube.
7. Heat the test tube for about 5 minutes.
8. Allow the test tube to cool for about 5 minutes.
9. Mass it. Never place a hot substance on the scale.
10. Repeat as needed until the mass remains constant.
DATA / CALCULATIONS / RESULTS
You will show your data table, calculations, and final result.
LAB REPORT
A formal write-up is not required for this lab.
Your lab report should include:
 Your name (# of week schedule)
 Name of lab
 DATA / CALCULATIONS / RESULTS
Submit to Dropbox folder - Lab: Percent Composition of a Mixture