The Law of Conservation of Matter

CHEM 1315 Lab 10: Conservation of Mass
Conservation of mass and writing ionic equations

Lab Description:

The Law of Conservation of Matter states that matter can neither be created nor
destroyed.

Writing Ionic Equations

Refer to Sections 4.2 and 4.3 in your textbook. Use Table 4.1.

Reactions between ionic species may lead to a “double displacement reaction” (a/k/a
metathesis). In an aqueous environment, one knows a reaction has occurred if a
precipitate (a solid), a liquid (usually water), or a gas forms.

A Generic Double Displacement Reaction has the form:

AX (aq) + BY (aq) → AY + BX (s, l, or g)

To decide whether a reaction has occurred, inspect each reactant species. If a reaction
occurs a new combination of cation and anions must occur.

A2+ B2+

X2- Y2-

The only possible re-combinations are AY and BX. (You always have a cation and an
anion in an ionic compound.)

One must evaluate whether AY or BX will reform or whether they will remain isolated ions
in the solution. In the following exercise, we will limit the analysis to the formation of a
precipitate (“ppt”).

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In Table 4.1, the left side rules concern “soluble ionic compounds.” This means that when
a solid compound is placed in water, the ions of the compound are separated and
isolated by water molecules. The polar water molecules help offset the charge of the
ions. These ions do not re-form with other soluble ions to form a new compound.

A reaction will occur between ions in an aqueous (water) environment if two “insoluble”
ions are combined though. The right side of Table 4.1 indicates the types of ions that are
“insoluble.”

Example Rules

• All carbonates (CO32-) are insoluble unless the cation is from Group IA or NH4+; for
example, Na2CO3, K2CO3, Li2CO3 are soluble compounds and BaCO3, Al2(CO3)3,
Ag2CO3, etc., are insoluble.

• Most chloride are soluble (see exceptions).

• All nitrate compounds are soluble.

Consider the following reaction:

Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2 Na+ (aq) + 2 Cl- (aq)

The soluble Na2CO3 will dissociate into the ions Na+ and CO32- in water. Likewise CaCl2
forms Ca2+ and Cl- ions.

Na+ Ca2+

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CO32- Cl-

Possible re-combinations:

CaCO3 and NaCl

Solid CaCO3 will form in the solution but Na+ and Cl- will remain hydrated ions and have
no tendency to reform.

Problem Description:

You are trying to explain two chemistry concepts to a fellow student. You will prove that
the law of mass conservation is correct. You will also show your fellow student how to
write molecular, total ionic, and net ionic equations. These equations will indicate to your
fellow student how ionic substances behave in water.

For these problems, please click
on the ‘Molarity” tab of the solution
info pane to display units in grams.

Experimental Procedure:

Procedure 1: Prove the Law of Mass Conservation

1. Place the Erhlenmeyer flasks with 1.00 g AgNO3 and 1.00 g NaCl on the benchtop.
Place the distilled water on the benchtop.

2. The solution labeled “1.00 g NaCl” contains 1.00 g of NaCl dissolved in water. Use
the solution viewer to determine the number of grams of Na+ and Cl- in the solution,
and confirm that these add to 1.00 g.

3. The solution labeled “1.00 g AgNO3” contains 1.00 g of solid AgNO3. Add 100.0 mL
of water to this solution by choosing the “Precise Transfer” mode, input 100.0 mL in
the “Transfer Amount” and then “Pour” it into the flask. Use the solution viewer to
determine the number of grams of Ag+ and NO3- in the solution.

4. Clear the flasks off the benchtop.

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Procedure 2

5. Place a new 1.00 g NaCl Erhlenmeyer flask and the solid AgNO3 on the benchtop.

6. Add 1.00 g of solid AgNO3 from the bottle of AgNO3 to the 1.00 g solution of NaCl
on your workbench. To do this, choose the “Precise Transfer” mode in your Tools
icon. Place the bottle of solid AgNO3 over the solution of NaCl and input 1.00 gram
into the “Transfer Amount.” “Pour” it into the solution. Write down the mass of each
species in solution and the mass of solid AgCl formed after the reaction occurs.

(Note: You can switch the solution viewer to “Solid” to determine the amount of solid
AgCl in the flask.) Confirm that the amounts of these species are consistent with what
you say in parts (1) and (2). In other words, confirm that the reactants and products have
the same mass.

Written Assignment:

Procedure 1

1. Identify the mass of each reactant ion and the total mass of the reactants.

2. Identify the mass of each product and the total mass of the product.

3. Explain how this experiment confirms the Law of Mass Conservation.

Procedure 2

1. Write a balanced molecular equation for the reaction of solid AgNO3 with aqueous
NaCl. Be sure to include the correct number of coefficients and the state of the
species (aq, s, l or g).

2. Write the total ionic equation for this reaction, including all physical states.

3. Write the net ionic equation for this reaction, including all physical states.

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4. Identify the spectator ions.

5. If the chloride ion is usually soluble, how can solid AgCl form? Identify the rules
from Table 4.1 and explain the physical state of the products. (Be sure to discuss
the exception to the chloride rule.)

Problems:

Using the Solubility Rules on page 143 in Table 4.1, write the balanced molecular, total
ionic, and net ionic equations (including physical states) for the following:

Hints: There is no such thing as an Na3+. Na is in group 1A, so you know it can only form
a +1 charge. It will form 3 Na+ ions for each Na3PO4. There is a diatomic molecule Cl2.
But when ionic compounds are dissolved in water, they form ions so the dissociation is to
2 Cl- from the MgCl2.

1. Na3PO4 (aq) + MgCl2 (aq) →

2. Al(CH3COO)3 (aq) + KOH (aq) →

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CHEM 1315

Lab 10: Conservation of Mass

Conservation of mass and writing ionic equations

Page 1 of 2

Name:

Written Assignment:

Procedure 1

  • Identify the mass of each reactant ion and the total mass of the reactants.
  • Identify the mass of each product and the total mass of the product.
  • Explain how this experiment confirms the Law of Mass Conservation.

Procedure 2

  • Write a balanced molecular equation for the reaction of solid AgNO3 with aqueous NaCl. Be sure to include the correct number of coefficients and the state of the species (aq, s, l or g).
  • Write the total ionic equation for this reaction, including all physical states.
  • Write the net ionic equation for this reaction, including all physical states.
  • Identify the spectator ions.
  • If the chloride ion is usually soluble, how can solid AgCl form? Identify the rules from Table 4.1 and explain the physical state of the products. (Be sure to discuss the exception to the chloride rule.)

Problems:

Using the Solubility Rules on page 143 in Table 4.1, write the balanced molecular, total ionic, and net ionic equations (including physical states) for the following:

Hints: There is no such thing as an Na3+. Na is in group 1A, so you know it can only form a +1 charge. It will form 3 Na+ ions for each Na3PO4. There is a diatomic molecule Cl2. But when ionic compounds are dissolved in water, they form ions so the dissociation is to 2 Cl- from the MgCl2.

  • Na3PO4 (aq) + MgCl2 (aq) →
  • Al(CH3COO)3 (aq) + KOH (aq) →
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