Lab 1: The Law of Definite Proportions
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LAB 1: THE LAW OF DEFINITE PROPORTIONS
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This lab does NOT require a lab report.
You will work with a partner for this lab.
I Introduction
In 1797, the French chemist Joseph Proust published a stunning revelation. In fact, it
was such an amazing epiphany that it goes by no less than three different names:
Proust’s Law, the Law of Definite Proportions, and the Law of Constant Composition.
Whatever you call it, it basically means the following:
A given compound (such as water or carbon dioxide) will always have the same
proportion of its elements, by mass, regardless of where it came from or how
it was made.
In this activity, you will confirm this fundamental law of chemistry with the simple
reaction between copper and iodine, which forms copper iodide (shown in the
following “word equation”):
Copper(s) + iodine(s) → copper iodide(s)
In addition, you will also practice using the analytical balance and making and
recording accurate and precise measurements.
Equipment and Reagents
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Bunsen burner and tongs
Metal spatula
Steel wool
Analytical balance
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Copper strip, Cu(s)
Iodine crystals, I2(s)
! Warnings!
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You will be working with a Bunsen burner and an open flame. Tie back any loose
hair or clothing.
Iodine is an irritant, inhalation hazard, and can stain skin and clothing. Wear an
apron and goggles.
When heated, the iodine sublimes into a pretty but choking purple vapor. Heat the
iodine in the fume hood only.
Lab 1: The Law of Definite Proportions
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Procedure
1. Clean a strip of copper with steel wool and wipe it off. Weigh and record the mass of
copper to at least three decimal places.
2. Using a spatula, place several iodine crystals on the copper strip such that the surface
is lightly covered.
3. Using tongs, carefully hold the strip and its balanced crystals in the lit Bunsen burner
in the hood. A voluminous cloud of purple vapor will be formed as excess iodine
rapidly sublimes (goes from the solid to the gaseous state).
4. Remove and observe the copper strip. Weigh and record its mass.
5. Scrape the copper iodide product from the strip and gently clean it with steel wool.
Weigh and record the mass of the freshly-cleaned copper strip.
6. Write your results on the board and record the results of your classmates.
7. Repeat, if time allows.
8. Verify the law of definite proportions.
Data and Calculations (to be recorded on your lab Data Sheet)
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Title and Purpose
Procedure used
Observations of reactants, reaction, and products
A copy of the following table:
Trial
Mass of
Original,
Clean Cu
strip
Mass of Cu
strip +
copper
iodide
product
Mass of
iodine in
product
Mass of
clean Cu
strip
Mass of
copper in
product
Ratio of
Iodine
mass to
Copper
mass
Trial 1
Trial 2
Etc.
? Questions to Answer
1) Why should the mass of the strip increase after the reaction with iodine?
2) Why should the mass of the clean, post-reaction copper strip be less than the
mass of the clean, pre-reaction copper strip?
3) The expected mass ratio of iodine:copper is 2.0:1.0.
a. What is your average value of iodine:copper?
b. What is your percent error?
4) What are some possible sources of error in this activity?
5) Did this activity confirm Proust’s law of definite proportions? Explain.