Stoichiometry of a Reaction
Qualitative analysis is the branch of chemistry that involves the identification of the substances
present in a sample, while quantitative analysis indicates the amounts of each substance
present. In this experiment you will perform a quantitative analysis technique that potentially
could be used for the determination of the amount of copper in a sample. Using a two-step
reaction sequence, copper(II) oxide will be synthesized from a weighed amount of copper(II)
sulfate pentahydrate.
Stoichiometry allows us to use the balanced equation to calculate the amounts of product and/
or reactant involved in a reaction. Because the starting material will be a pure compound, it will
be possible - to check the validity of the technique by comparing the amount of copper(II) oxide
predicted theoretically to the amount experimentally obtained.
Copper(II) pentahydrate will be dissolved in water and reacted using a double replacement
reaction with sodium hydroxide. The addition of hydroxide ions to a solution containing
copper(II) ions results in the precipitation of copper(II) hydroxide.
CuSO4 + 2 NaOH Cu(OH)2(s) + Na2SO4
Subsequent heating of the copper(II) hydroxide results in its decomposition to copper(II) oxide
and water.
Cu(OH)2(s) + CuO(s) + 2 H2O
The CuO can be quantitatively filtered, dried and weighed. The overall reaction for the
sequence is:
CuSO4•5H2O(s) + 2 NaOH(aq) CuO(s) + Na2SO4(aq) + 6 H2O(l)
You will perform the sequence above with an accurately weighed amount of CuSO 4•5H2O.
From this amount it is possible to calculate the amount of copper(II) oxide that should be
formed (the theoretical yield). By performing the experiment, the experimental (actual) yield is
obtained and this value is compared with the theoretical yield. Thus the %yield is
%yield =
actual yield
theoretical yield
x 100
A percent yield close to 100% indicates that the technique is sound and could be used as a
quantitative analysis technique for copper.
