Formula of an Ionic Compound
Balancing Charges of Ions
Background:
Atoms of different elements combine with one another to form
compounds. The formula of the compound indicates the kinds of atoms
present and the relative number of each atom. Today you will learn how the
formula of an ionic compound can be determined experimentally.
Remember that ionic compounds are composed of ions–atoms or
groups of atoms that have a positive or negative charge. Atoms that lose
electrons carry a positive charge and are called cations. Atoms that gain
electrons carry a negative charge and are called anions. Oppositely charged
ions arrange themselves into an extended three-dimensional structure called a
crystal lattice. The attractive forces between oppositely charged ions are
called ionic bonds. Although composed of charged ions, ionic compounds are
electrically neutral. That is, the positive charges exactly balance the negative
charges.
Many ionic compounds can be prepared in the lab using precipitation
reactions. When solutions of two ionic compounds are combined, the ions
may rearrange to form new ionic compounds that stick together and form a
solid. An example of this type of reaction occurs when barium chloride (BaCl2)
reacts with sodium sulfate (Na2SO4) to form the insoluble solid barium sulfate
(BaSO4) and sodium chloride (NaCl). The equation below details the reaction.
Notice that the letters in parentheses tell you whether the compound is
dissolved in the solution (aq) or is a precipitate (s).
BaCl2 (aq) + Na2SO4 (aq)  BaSO4 (s) + 2 NaCl (aq)
According to the balanced equation above, barium cations (Ba2+) react
with sulfate anions (SO42-) in a 1:1 ratio to form barium sulfate (BaSO4). This
ratio can be observed in the lab by mixing BaCl2 (aq) and Na2SO4 (aq)
containing equal amounts (concentrations) of barium and sulfate ions,
respectively. The maximum amount of precipitate will be formed when the
equal volumes (1:1 ratio) of the two solutions are combined. A similar
approach can be used to determine the formula of an unknown ionic
compound.
Pre Lab Questions:
1. What is a precipitate?
2. What does soluble and insoluble mean?
3. What are anions and cations?
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Read the procedure below and then answer these remaining questions.
4. What is the name of the precipitate that will form in today’s lab?
5. What is the color of the solutions that you are using today?
a. sodium phosphate
b. copper chloride
6. The blue color of the copper chloride solution is evidence of
( copper cations / chloride anions ).
7. On the next page of your lab notebook, write the title of the lab, the
question we are trying to answer, and then summarize steps 1-6. Draw
the data table below.
8. During the lab you will need to measure the mass of the copper phosphate
precipitate. What lab technique is used to separate a solid from a liquid?
a. In order to measure the mass of just the solid, what mass do you need
to record prior to filtering the solid?
Procedure:
1. The precipitate in the today’s reaction is copper phosphate. You will
determine the ratio of copper cations and phosphate anions in copper
phosphate.
a. Sodium phosphate solution will act as your source of phosphate (PO4)
anions, and the copper chloride solution will act as your source of
copper cations.
2. Using labeling tape, label each test tube 1-6.
3. Place 2mL of phosphate anions in each of the six test tubes.
4. Add the appropriate amount of copper cations to each test tube.
Test tube #
1
2
3
4
5
6
5. Swirl each test tube to mix the
Volume of Cu ions
1
2
3
4
5
6
cations (mL)
reactants.
Volume of PO4 ions
2
2
2
2
2
2
6. Before you continue, predict which
anions (mL)
Ratio of copper cations
test tube should have the greatest
to phosphate anions
amount of precipitate. Explain your
Color of liquid
after filtering
reasoning.
7. In your lab notebook, describe a procedure for measuring the mass of the
copper phosphate precipitate.
a. Draw a data table in your lab notebook for you mass data. (Hint: You will
need space to write three masses for each test tube.)
8. Get your procedure approved before continuing the lab.
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