176 DIY Science: Illustrated Guide to Home Chemistry Experiments
SCHIToI omETRy
Stoichiometry is used to calculate quantitative
relationships between the reactants and products
in a chemical reaction. Stoichiometry is based
upon the Law of Conservation of Mass, the Law of
Constant Proportions (also called the Law of Definite
Proportions), and the Law of Multiple Proportions.
In most chemical reactions, the reactants and products
combine in fixed proportions. For example, one molecule
of reactant A may combine with two molecules of
reactant B to yield one molecule of product C and two
molecules of product D. Because a chemical reaction
can neither create nor destroy matter, nor transmute
one element into another, the total number of atoms of
each element present in the reactants must equal the
total number of atoms of each element present in the
products.
Chemists use stoichiometry to balance equations—as
we did for the reaction of copper(II) sulfate and sodium
hydroxide—and to specify the mole ratio (or molar
proportions) of elements in a compound. For example,
the mole ratio of copper:sulfur:oxygen in copper(II)
sulfate is 1:1:4.SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a large Pyrex saucer or similar
heat-resistant container for the evaporating dish. - You may substitute for the hotplate another heat
source, such as a kitchen stove burner or alcohol
lamp with wire gauze and stand. - For this laboratory session, we’ve specified quantities
of chemicals sufficient to make a useful amount of
Schweizer’s Reagent. If you do not intend to make
or use the Schweizer’s Reagent, you may reduce
the quantities of chemicals to one-tenth (2.50 g of
copper(II) sulfate pentahydrate and 0.80 g of sodium
hydroxide) and substitute test tubes or similar
containers for the beakers. - You may substitute 15.96 g of anhydrous
copper(II) sulfate for the 24.97 g of copper(II)
sulfate pentahydrate. You may also substitute raw
copper sulfate from the hardware store (sold as
root killer) or the copper sulfate you purified by
recrystallization in an earlier laboratory session
Lab 6.3, “Recrystallization: purify Copper Sulfate”.
POCEDURER
PART I:
This laboratory has two parts. In Part I, we’ll react solutions of
copper(II) sulfate and sodium hydroxide to produce copper(II)
hydroxide and sodium sulfate, determine the masses of the
products, and calculate the stoichiometric relationships for the
reaction. In Part II, we’ll use the copper(II) hydroxide we produced
in Part I to synthesize Schweizer’s Reagent.- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Weigh about 24.97 g of copper(II) sulfate pentahydrate,
and record the mass to 0.01 g on line A of Table 9-4. - Calculate the number of moles of copper(II) sulfate
represented by this mass, and record the number of
moles on line B of Table 9-4. - Transfer the copper sulfate to a 150 mL beaker, add
about 100 mL of water, and stir until the copper sulfate
dissolves completely. - Weigh about 8.00 g of sodium hydroxide, and record the
mass to 0.01 g on line C of Table 9-4. - Calculate the number of moles of sodium hydroxide
represented by this mass, and record the number of
moles on line D of Table 9-4. - Transfer about 10 mL of water to the second 150 mL
beaker, and add the sodium hydroxide to the water with
constant stirring or swirling. (Caution: this reaction is
extremely exothermic.) - With constant stirring, pour the sodium hydroxide
solution into the beaker of copper(II) sulfate solution.
Rinse the sodium hydroxide beaker two or three times
with a few mL of water and transfer the rinse water to the
beaker of copper sulfate solution to make sure that you’ve
transferred all of the sodium hydroxide (Figure 9-5). - Rinse the empty beaker thoroughly and set up your
funnel holder and funnel with the empty beaker as the
receiving container.
Fanfold a piece of filter paper, weigh it, record its mass to
0.01 g on line E of Table 9-4, and place the filter paper in
the funnel.
Stir or swirl the beaker that contains the copper(II)
hydroxide to make sure as much as possible is
suspended in the solution, and then pour the solution
into the filter funnel, catching the filtrate in the empty
150 mL beaker. Rinse the reaction beaker two or three
times with a few mL of water and transfer the rinse water
into the filter funnel to make sure all of the copper(II)
hydroxide is retained by the filter paper.
Squirt a few mL of water onto the retained copper(II)
hydroxide in the filter paper to rinse as much of the soluble
sodium sulfate as possible into the receiving container.