AP Lab #4: Formula of a Hydrate
Purpose: Determine the empirical formulas of several hydrates by making anhydrous crystals through
drying.
Pre-lab:
1. Make the following chart in your lab notebook:
Substance #1
Substance #2
Mass of empty pipet (g)
Mass of hydrate & pipet (g)
Mass after 1st heating (g)
Mass after 2nd heating (g)
Mass after 3rd heating (g)
Constant mass of anhydrous salt & pipet (g)
Mass of water driven off (g)
Number of moles of water
Formula weight of anhydrous salt (g)
Grams of anhydrous salt (g)
Moles of anhydrous salt
# moles of water / # moles of anhydrous salt
Empirical formula of hydrate – experimental
Empirical formula of hydrate – theoretical
2. EMPIRICAL FORMULA QUESTIONS
Materials:
 Alcohol Burner





Wire Gauze
Ring Stands
Pasteur pipets
Balance
Unknown Hydrate
Procedure:
CAUTION: In the lab you will be heating glass – hot glass looks the same as cold glass! Be careful!
1. Obtain and mass a dry Pasteur pipet. Use the same balance throughout the rest of the experiment.
2. Fill the pipet with approximately 0.50 grams of an unknown hydrate (it does not need to be exact, but
pretty close). This can be done by trial and error until you have about the correct amount. Be sure to
distribute the hydrate along the length of the pipet. Try to avoid packing the crystals in too tightly.
3. Mass the pipet with the crystals.
4. Lay the pipet on a square of wire gauze that is sitting on a ring on a ring stand. Use a wire gauze square
that does not have a ceramic center. If there are only wire gauzes with ceramic center’s left, lay the
pipet along the edge so that the flame from the alcohol lamp can heat the entire length of the pipet.
5. Light your alcohol lamp. Heat the pipet by gently waving the Bunsen burner flame back and forth
underneath the wire gauze. Write down any observations in your lab notebook that may occur as you
heat the hydrate.
6. Heat for approximately 3 minutes. Allow the pipet to cool before handling.
7. Mass the pipet.
8. Heat again for 3 minutes. Repeat steps 5-7 until the mass of the hydrate is constant.
9. Repeat steps 1-8 with a sample of a different type of hydrate.
LABS
Analysis:
1. Record your empirical formula for the two hydrates on the whiteboard. Copy down the class
information into your notebook.
2. Record the actual empirical formula provided by Mr. Lance after the lab is complete.
Conclusion Questions:
1. Were your empirical formulas correct? If not, what are some possible sources of error that could
have occurred during the lab and how would that have affected your results. If so, what did
you do that limited your sources of error in the lab and had they occurred, how would the
errors have affected the outcome.
2. Suggest two reasons why the procedure used in this experiment might not be suitable for all
hydrates?
Formal Lab Report #1: Empirical Formula of a Hydrate
For this lab, you will be writing your first formal lab report for chemistry. Your formal lab report
must contain the following sections:
 Introduction
 Procedure
 Data/Results
 Discussion
Remember, the introduction should provide an introduction to the ideas that were covered in this
lab, a discussion of the purpose/focus of your experiment, and a hypothesis for the lab (this should
be in the form of an if/then statement). The discussion section should contain a paragraph with a
conclusion that you can draw (using your data) and answers to the two conclusion questions found
on this handout. Please be careful not to plagiarize. While you are encouraged to use the ideas on
this handout, under no circumstances can you copy the words or paraphrase the ideas shown here.
The formal lab report will be due Monday/Tuesday September
24th/25th
LABS