Discussion of Theory:
State the purpose of the lab. Two basic experimental procedures were carried out to fulfill the purpose of the
lab—(a) the hydrate was heated and (b) aluminum metal was added to the copper chloride solution. Give a
basic description of each of these procedures, explain concepts and scientific terms of the procedures, and
explain how the procedures allowed the purpose of the lab to be fulfilled.
Note: Key terms needed: hydrate, anhydrous, chemical reaction, empirical formula.
You must write the balanced chemical equation for any chemical reactions involved.
In this lab, the empirical formula for a copper chloride hydrate (CuxCly · z H2O) was to be found. To
accomplish this, the hydrate was first heated to drive off the water of hydration, leaving behind the
anhydrous crystal, CuxCly. By difference, the mass of H2O was found. Next, the anhydrous crystal was
dissolved and reacted with aluminum by the reaction
3 Cu2+ (aq) + 2 Al (s) → 2 Al3+ (aq) + 3 Cu (s)
The pure copper was collected, dried, and massed, and the mass of Cl was then determined by difference
from the anhydrous sample. Finally, all masses were converted to moles and compared to the moles of Cu
(the only material to be determined directly) to determine the empirical formula of the hydrate.
Experimental Sources of Error:
1) Accuracy: State the % error for moles of chlorine in formula and % error for moles of water in formula.
State whether your values were higher or lower than the accepted values.
Answers vary.
2) Precision: State the number of significant figures in each of the measurements taken and state which
measurement limits your precision.
The only measurements in this lab are mass measurements. The balance is precise to 0.001 g, and mass
differences typically are less than 1 g, leading to a precision of 3 significant figures.
3) Discussion of possible systematic errors/limitations of the lab design. There are really two measurements in
this lab that are prone to systematic error/limitations—the mass of the anhydrous crystal and the mass of
copper.
a) If your mass of anhydrous crystal was too low (assume mass of copper is correct), would your value
for the moles of chlorine in the formula be too high or too low? Would your value for the moles of
water in the formula be too high or too low?
If the mass of the anhydrous crystal is too low while the copper is correct, the mass of the chlorine
will also be low, and the ratio of the moles of Cl to moles of Cu will be low. Since the anhydrous
mass is used to find the mass of water, the mass of the water will be high, and so will the moles of
water and the mole ratio of H2O/Cu.
b) If your mass of copper was too low (assume mass of anhydrous is correct), would your value for the
moles of chlorine in the formula be too high or too low? Would your value for the moles of water
in the formula be too high or too low?
If the mass of copper is low but the anhydrous crystal is correct, then the moles of Cl will be high
since it is determined by subtracting the low mass of copper from the correct mass of the
anhydrous. The mole ratio of Cl/Cu, which is the subscript in the formula, will thus also be low.
Although the absolute mass and moles of H2O will be correct, since they will be compared to a low
value of the moles of Cu the ratio of moles H2O/Cu will be too high.
c) For the mass of the anhydrous crystal, state two systematic errors/limitations of the lab design. For
each error, state whether the error would give rise to a high or low mass of anhydrous and explain
how one could change the procedure to minimize the occurrence of the error.
Some possible answers include:
• Overheating would convert CuCl2 to CuO and lead to a low mass of the anhydrous crystal. Slow,
gentle heating while mixing around the crystals could minimize this error.
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Starting with a crucible that is not completely dry would also lead to a low mass of the
anhydrous crystal. Drying and massing the crucible to a constant mass before starting would
minimize this error.
Allowing the crystal to cool after heating the hydrate without placing the lid quickly enough on
the crucible could allow partial rehydration of the anhydrate, which would lead to a high mass of
anhydrate. Placing the lid on the crucible before removing the heat could minimize this error.
Not allowing the crucible and anhydrate to fully cool prior to massing would lead to convection
currents giving a low mass of the anhydrous crystal. Allowing adequate time to reach room
temperature could minimize this error.
Not allowing the empty crucible to cool fully prior to massing would lead to a lower crucible
mass, thus higher hydrate and anhydrate mass. Allowing adequate time to reach room
temperature could minimize this error.
Note: the procedure contains steps to ensure that no hydrate remains after heating; a different
method would need to be suggested for credit.
d) For the mass of the copper solid, state two systematic errors/limitations of the lab design. For each
error, state whether the error would give rise to a high or low mass of copper and explain how one
could change the procedure to minimize the occurrence of the error.
• If the size of the copper crystals formed was too fine, then some copper would pass through the
filter paper, leading to a low mass of copper. Using a finer grade of filter paper or using a
process that leads to larger copper crystal could minimize this error.
• Removing the filter paper from the Büchner funnel could lead to loss of copper due to the
difficulty of separating it from the funnel. Taking great care in collecting all of the copper or
using special filter paper with sides to contain the contents (which doesn’t exist, to my
knowledge) could minimize this error.
• If the copper were not completely dried, then the measured mass of copper would be too high.
Drying and massing the copper several times until a constant mass was obtained would assume
that all the water was removed and only the copper remained.
• Adherence of the copper to the aluminum wire would reduce the collected mass of copper.
Extra steps to oxidize the remaining aluminum would allow more copper to be collected and
could minimize this error.