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24 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
3
Conservation of Mass Over two centuries ago, chemists established a fundamental law called the law of conservation of mass.
This law states that during a chemical reaction, mass is neither lost
nor gained. In other words, all the matter present at the start of a
reaction still exists at the end of the reaction. The law of conservation of mass can be stated in mathematical form as follows.
Mass reactants ϭ Mass products
Example Problem 3-1
Law of Conservation of Mass
A thin strip of iron with a mass of 15.72 g is placed into a solution
containing 21.12 g of copper(II) sulfate and copper begins to form.
After a while, the reaction stops because all of the copper(II) sulfate
has reacted. The iron strip is found to have a mass of 8.33 g. The
mass of copper formed is found to be 8.41 g. What mass of iron(II)
sulfate has been formed in the reaction?
Solution Apply the law of conservation of mass.
In this reaction, there are two reactants and two products, so the law
of conservation of mass can be restated as follows.
Mass reactant 1 ϩ Mass reactant 2 ϭ Mass product 1 ϩ Mass product 2
Rewrite the equation with the names of the reactants and products.
Mass iron ϩ Mass copper sulfate ϭ Mass copper ϩ Mass iron sulfate
To find the mass of iron sulfate, rearrange the equation.
Mass iron sulfate ϭ Mass iron ϩ Mass copper sulfate Ϫ Mass copper
Then, determine the mass of iron that reacted.
Mass iron ϭ original mass of iron Ϫ mass of iron remaining
Mass iron ϭ 15.72 g Ϫ 8.33 g ϭ 7.39 g
Finally, substitute the masses into the equation and solve.
Mass iron sulfate ϭ 7.39 g ϩ 21.12 g Ϫ 8.41 g ϭ 20.10 g iron sulfate
To check your work, make sure the sum of the masses of the
reactants is equal to the sum of the masses of the products.
▲
24 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
3
Conservation of Mass Over two centuries ago, chemists established a fundamental law called the law of conservation of mass.
This law states that during a chemical reaction, mass is neither lost
nor gained. In other words, all the matter present at the start of a
reaction still exists at the end of the reaction. The law of conservation of mass can be stated in mathematical form as follows.
Mass reactants ϭ Mass products
Example Problem 3-1
Law of Conservation of Mass
A thin strip of iron with a mass of 15.72 g is placed into a solution
containing 21.12 g of copper(II) sulfate and copper begins to form.
After a while, the reaction stops because all of the copper(II) sulfate
has reacted. The iron strip is found to have a mass of 8.33 g. The
mass of copper formed is found to be 8.41 g. What mass of iron(II)
sulfate has been formed in the reaction?
Solution Apply the law of conservation of mass.
In this reaction, there are two reactants and two products, so the law
of conservation of mass can be restated as follows.
Mass reactant 1 ϩ Mass reactant 2 ϭ Mass product 1 ϩ Mass product 2
Rewrite the equation with the names of the reactants and products.
Mass iron ϩ Mass copper sulfate ϭ Mass copper ϩ Mass iron sulfate
To find the mass of iron sulfate, rearrange the equation.
Mass iron sulfate ϭ Mass iron ϩ Mass copper sulfate Ϫ Mass copper
Then, determine the mass of iron that reacted.
Mass iron ϭ original mass of iron Ϫ mass of iron remaining
Mass iron ϭ 15.72 g Ϫ 8.33 g ϭ 7.39 g
Finally, substitute the masses into the equation and solve.
Mass iron sulfate ϭ 7.39 g ϩ 21.12 g Ϫ 8.41 g ϭ 20.10 g iron sulfate
To check your work, make sure the sum of the masses of the
reactants is equal to the sum of the masses of the products.
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