Law of Multiple Proportions
Abstract:
Many elements form more than one compound with oxygen or other nonmetals. The law of
multiple proportions states that in such cases the different GEW's of the element in question are in a
ratio of simple whole numbers. This experiment will illustrate the validity of the law of multiple
proportions and give more experience with simple reactions and lab procedures, through carrying a
know mass of copper bromide through conversions to another copper bromide compound, copper
oxide, and finally pure copper.
Procedure:
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Weigh empty test tube
Obtain 1.0g sample Copper Bromide
Weigh test tube and Copper Bromide to .001g
Set up apparatus (lab manual fig 5.1)
a) 500 ml flask (Florence)
b) Glass tube extending 1” beyond stopper out of test tube, 1” from water
Heat test tube gradually, then strongly
a) Excessive heat will break Copper Bromide down too much!
b) When Br2 is no longer evolved, heat upper end of test tube to drive out Bromine
Let test tube cool, then weigh
Add 2ml 15M HNO3 Keep under hood!
Reassemble apparatus
Heat tube gently until copper nitrate breaks down to copper oxide. Let cool
Weigh test tube
Set up hydrogen generator as per lab #3
Reduce CuO to Cu
a) Remember to burn off water
b) Be careful of H2/O2 mixture. Use test tube to light.
Allow test tube to cool while keeping H2 flowing through system.
When test tube is near 25º, weigh tube and contents
Break down apparatus, clean equipment and area and vamoose!
Addenda:
a) Save bromine water and put in flask in fume hood
b) When heating test tubes watch for breakage, immediately cover everything with a towel
c) NHO3 only from Welch or MacFarlane.
d) Use 2g Copper Bromide instead of 1g
e) Turn all copper oxide back to Welch
f) You will enjoy this lab!
Law of Multiple Proportions
page 2
Don’t Forget Observations!!!!
Data:
Weight of empty test tube
_________g
Weight of test tube plus copper bromide(A) ________g
Weight of test tube plus copper bromide (B) _________g
Weight of test tube plus copper oxide (C)
____g
Weight of test tube plus copper
_________g
Wight of copper bromide (A)
_____g
Weight of copper bromide (B)
_____g
Weight of copper oxide
__________g
Weight of copper
_____g
Law of Multiple Proportions
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Calculations:
1. Using the data you obtained and the definitions of gram equivalent weight, calculate the GEW
of copper in the oxide (C).
GEW =
GEW Cu ___g
2. Assuming that on GEW Cu combines with one GEW Br in any copper bromide compound,
calculate the GEW Br in copper bromide (A) and copper bromide (B).
BrA =
GEW Br in (A) __g
BrB =
GEW Br in (B) ___g
3. How do these results illustrate the Law of Multiple Proportions?
Law of Multiple Proportions
page 4
4. Given the atomic weights for copper, bromine, and oxygen, find the chemical formulas for
compounds A, B and C.
Copper bromide (A) ___
Copper bromide (B) ___
Copper oxide (C)
____
5. Write balanced chemical equations for the reactions that occurred when
Compound A was heated.
Compound C was heated in a stream of hydrogen.
Note:
The calculations in this experiment are based on a constant gram equivalent weight for copper
in its compounds. This leads to a set of GEW's for other elements which differ to some extent from
those obtained using the usual assumptions that the gram equivalent weight of oxygen is constant. The
laws of chemistry are valid on either basis, and in this experiment it is certainly most convenient to
assume, as we do, that the GEW of copper remains fixed.