LAB – GRAVIMETRIC ANALYSIS
CONTEXT
Frequently chemists need to determine the specific amount of a particular substance in a
sample. Gravimetric analysis is a technique through which the amount of an analyte (the ion
being analyzed) can be determined through the measurement of mass. In gravimetric
analysis, the chemist reacts the sample containing the analyte with another compound that
will form a precipitate containing the analyte. When the analyte ion is formed into a
precipitate, it can be purified, collected through filtration, then dried and massed. From the
mass and the known composition of the precipitate, the amount of the analyte in the original
solution can be calculated stoichiometrically. When done properly, gravimetric analysis
provides an extremely precise quantitative analysis of the analyte.
In this lab you will learn the technique of gravimetric analysis. You will make a solution of
calcium chloride and then analyze that solution to determine the amount of calcium present.
Hopefully you are thinking - Why do I need to analyze the solution to determine its
contents if I made the solution myself? Wouldn’t I already know the contents?
The answer is yes! But, we are using this lab to learn the technique of gravimetric
analysis. You will combine your calcium chloride solution with a solution of sodium
carbonate. They will react to form solid calcium carbonate and sodium chloride
solution. You will perform the gravimetric analysis to isolate the solid calcium
carbonate. You will use stoichiometry to determine that amount of calcium
carbonate that should have formed and compare it with the amount that you actually
formed. If you perform the lab correctly, you should be able to recover the full
amount!
In the lab after this one you will analyze an actual unknown solution. So, use this lab
to perfect your technique!
MATERIALS:
Chemicals
Calcium chloride, (CaCl2)
Sodium carbonate, (Na2CO3)
Consumables
Filter paper
Student Provided
Distilled water
Equipment
Beaker
Graduated cylinder
Funnel
Stand
Clamp
Stirring rod
Watch glass
Balance
Metal scoop
Wash Bottle (for distilled water)
Drying oven (optional)
You may use any of the containers in your kit as needed.
SAFETY and DISPOSAL
All solutions can be flushed down the drain. Normal laboratory precautions should be taken,
including wearing goggles at all times. Information about each chemical can be found in the
safety section.
PROCEDURE
You will make a solution of sodium carbonate and mix it with a solution of calcium chloride.
Read the entire procedure before you begin. Create a data table for the data you will need to
collect.
1. Mass about 2 g of calcium chloride in a second clean, dry beaker.
2. Add about 20 mL of distilled water. Stir until the solid is dissolved.
This solution represents the unknown solution that you will test.
3. Mass about 2 g of sodium carbonate in a clean, dry beaker.
4. Add about 20 mL of distilled water to the beaker of sodium carbonate. Stir until the
solid is dissolved.
This is the solution you will add to your “unknown” to precipitate the analyte.
5. Pour a small amount of the sodium carbonate solution into the beaker containing the
calcium chloride solution. Stir and observe the mixture. Add some more of the sodium
carbonate solution, while stirring and observing. After all of the sodium carbonate
solution has been added, continue to stir this mixture for another couple of minutes.
6. Mass a piece of filter paper.
7. Set up the filtering apparatus. Refer to the lab techniques section.
In a more sophisticated laboratory, filtration like this would be done using vacuum
filtration, as it is much faster. We will be using gravity filtration. The process may be
slow, so be patient. You may leave the setup to allow the filtrate to flow if necessary.
Just make sure to keep it in a safe place, free from dust or debris.
8. Insert the filter paper into the filtering apparatus. Wet the filter paper with deionized
water.
9. Pour the contents of your beaker slowly into the funnel, using the stirring rod as a
guide to prevent splattering. Be careful that none of the mixture flows over the filter
paper or the funnel as you pour. Use a wash bottle of distilled water to rinse the
precipitate out of the beaker with small quantities of water. Use a little more distilled
water to wash the precipitate that is now collected in the filter paper.
10. Mass a clean, dry watch glass.
11. Carefully remove the filter paper with the precipitate from the filtering apparatus and
set it on the watch glass. The filtrate can be poured down the drain.
12. You may choose to let your sample dry in time or you may place the watch glass and
contents in a tow temp oven. Place the watch glass with the filter paper on it on a
cookie sheet or oven save pan. Place this into your kitchen oven set between 110°C
and 120°C for 10-15 minutes.
13. Carefully remove the warm watch glass, using a pot holder if necessary. Use a clean,
dry metal scoop to break the precipitate into small pieces.
14. Return the watch glass to the drying oven for another 5 minutes.
15. Mass again.
16. Continue until your sample is completely dry.
CALCULATIONS
1. Write the balanced equation for the reaction that occurred.
2. Use the original masses of sodium carbonate and calcium chloride to determine the
mass of calcium carbonate that could be formed in your experiment. Show all your
work and highlight or indicate your final answer in some way.
3. Calculate % yield of calcium carbonate. Show all your work and highlight or indicate
your final answer in some way.
POST-LAB QUESTIONS
1. After the filtration, how did you know when the precipitate was dry?
2. If one more gram of sodium carbonate were used, how would it affect the amount of
calcium carbonate that you calculated would form?
3. Would the mass of precipitate that you measured be larger or smaller if you did not
wash the precipitate before drying it?
4. How would your outcome change if the precipitate were massed without drying?
Explain.
5. Calculate the % error.
6. Explain the reasons for any unreasonable error. Be specific, including how you can
change your methods to avoid the same error next time. (Note – the amount of
unreasonable error will be different for each lab procedure. The amount of expected
product for this particular procedure was mentioned earlier in the lab. Possible errors
have also been mentioned. Go back and reread the lab if you are stuck here!)
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 table / Observations
 Calculations
 Answers to Post-lab Questions
Submit to Dropbox folder - Lab: Gravimetric Analysis