Name:_____________________________ Date:___________ Period:___
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
= Mass
products
Example Problem 3-1
Law of Conservation of Mass
A thin strip of iron (Fe) with a mass of 15.72 g is placed into a solution containing
21.12 g of copper(II) sulfate (CuSO4)and copper (Cu) 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 (FeSO4) 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.
Massreactant 1 + Massreactant 2 = Massproduct 1+ Massproduct 2
Rewrite the equation with the names of the reactants and products.
Massiron + Masscopper sulfate = Masscopper + Massiron sulfate
To find the mass of iron sulfate, rearrange the equation.
Massiron sulfate = Massiron + Masscopper sulfate - Masscopper
Then, determine the mass of iron that reacted.
Massiron = original mass of iron + mass of iron remaining
Massiron = 15.72 g = 8.33 g + 7.39 g
Finally, substitute the masses into the equation and solve.
Massiron sulfate = 7.39 g + 21.12 g - 8.41 g - 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.
Solve the following problems. Write the reactants and products. Clearly indicate
answers.
1. A 12.2 g sample of X reacts with a sample of Y to form 78.9 g of XY. What is
the mass of Y that reacted?
2. In the complete reaction of 22.00 g of sodium (Na) with 35.45 g of chlorine
(Cl2), what mass of NaCl is formed?
3. A 10.0 g sample of magnesium (Mg) reacts with oxygen (O2) to form 16.6 g of
magnesium oxide (MgO). How many grams of oxygen reacted?
4. A 28.0 g sample of nitrogen (N2) gas combines completely with 6.0 grams of
hydrogen (H2) gas to form ammonia (NH3). What is the mass of ammonia formed?
5. In a flask, 10.3 grams of aluminum (Al) reacted with 100.0 g of liquid bromine
(Br) to form aluminum bromide (AlBr3). After the reaction, no aluminum remained,
and 8.5 grams of bromine remained unreacted. How many grams of bromine
reacted? How many grams of compound were formed?
6. A student carefully placed 15.6 g of sodium (Na) in a reactor supplied with an
excess quantity of chlorine (Cl2) gas. When the reaction was complete, the
student obtained 39.7 g of sodium chloride (NaCl). Given that all of the sodium
reacted. How many grams of chlorine gas reacted and how many grams of chlorine
gas remained unreacted?
7. From a laboratory process designed to separate water into hydrogen (H2) and
oxygen (O2) gases, a student collected at the end of the process 10 g of hydrogen
and 79.4 g of oxygen. How much water was originally involved in the process?
8. A substance breaks down into its component elements when it is heated. If
68.0 g of the substance is present before it is heated, what is the combined mass of
the component elements after heating?
9. A sealed glass tube contains 2.25 g of copper(Cu) and 3.32 g of sulfur (S). The
mass of the tube and its contents is 18.48 g. Upon heating, a reaction forms
copper(II) sulfide (CuS). All of the copper reacts, but only 1.14 g of sulfur reacts.
Predict what the mass of the tube and its contents will be after the reaction is
completed. Explain your reasoning.
10. When heated, calcium hydroxide [Ca(OH)2] and ammonium chloride (NH4Cl)
react to produce ammonia (NH3) gas, water vapor (H2O), and solid calcium chloride
[Ca(Cl)2]. Suppose 5.00 g of calcium hydroxide and 10.00 g of ammonium chloride
are mixed in a test tube and heated until no more ammonia is given off. The
remaining material in the test tube has a mass of 10.27 g. What total mass of
ammonia and water vapor was produced in the reaction?
11. When a solution of barium nitrate [Ba(NO3)2] and a solution of copper(II)
sulfate (CuSO4) are mixed, a chemical reaction produces solid barium sulfate
(BaSO4), which sinks to the bottom, and a solution of copper(II) nitrate [Cu(NO3)2].
Suppose some barium nitrate is dissolved in 120.00 g of water and 8.15 g of
copper(II) sulfate is dissolved in 75.00 g of water. The solutions are poured
together, and a white solid forms. After the solid is filtered off, it is found to have a
mass of 10.76 g. The mass of the solution that passed through the filter is 204.44
g. What mass of barium nitrate was used in the reaction?
12. A reaction between sodium hydroxide (NaOH) and hydrogen chloride (HCl) gas
produces sodium chloride (NaCl) and water. A reaction of 22.85 g of sodium
hydroxide with 20.82 g of hydrogen chloride gives off 10.29 g of water. What mass
of sodium chloride is formed in the reaction?