Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Conservation of Mass starts by naming reactants and products, then checks conservation with a balanced equation.
Common stuck point:Students often know a formula related to conservation of mass but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?
Common Mistakes to Watch For
Before you work through the examples, skim the mistake guide so you know which shortcuts and
sign errors to avoid.
If 10 g of hydrogen reacts with 80 g of oxygen to form water, what is the total mass of water produced?
Answer
90g of water
First step
1
State the law of conservation of mass: in a chemical reaction, mass is neither created nor destroyed.
Full solution
2
Add up the masses of all reactants: 10g+80g=90g.
3
By conservation of mass, the total mass of products must also equal 90g.
Antoine Lavoisier established that in any chemical reaction, the total mass of the reactants equals the total mass of the products. This fundamental law underpins all of stoichiometry.
Example 2
medium
In a sealed container, 50.0 g of calcium carbonate decomposes: CaCO3→CaO+CO2. If 28.0 g of CaO is produced, what mass of CO2 is released?
Example 3
medium
If 50 g of Pb(NO3)2 (in solution) reacts with 30 g of KI (in solution) and 35 g of PbI2 precipitate forms, what is the mass of the solution (KNO3) left over?
Example 4
hard
Mercury(II) oxide decomposes: 2HgO→2Hg+O2. From 21.7 g of HgO, 20.1 g of Hg forms. What mass of O2 was released?
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easy
A student burns 12 g of magnesium in air and obtains 20 g of magnesium oxide. How much oxygen reacted?
Example 2
easy
If 12 g of magnesium reacts with oxygen to form 20 g of magnesium oxide in a closed container, how many grams of oxygen reacted?
Example 3
easy
10 g of A reacts completely with 15 g of B in a sealed flask. What is the total product mass?
Example 4
easy
Why does mass stay constant in a chemical reaction?
Example 5
easy
In a sealed flask, 50 g of reactants react. What do the products weigh?
Example 6
easy
Does the number of moles always stay the same in a reaction?
Example 7
easy
If a candle burns in a sealed jar, does the jar's total mass change?
Example 8
easy
A reaction in an open beaker shows decreasing mass. What likely happened?
Example 9
easy
Magnesium burns and the product (ash) weighs MORE than the metal. Why is mass still conserved?
Example 10
easy
Who is credited with establishing the law of conservation of mass?
Example 11
medium
4 g H2 reacts with 32 g O2 to form water. What mass of water forms?
Example 12
medium
Methane burns: 16 g CH4 reacts with O2 to give 44 g CO2 and 36 g H2O. What mass of O2 reacted?
Example 13
medium
In 2H2+O2→2H2O, 4 g H2 reacts with 32 g O2. Show mass is conserved if 36 g water forms.
Example 14
medium
28 g of nitrogen reacts with 6 g of hydrogen to make ammonia. What mass of ammonia forms (sealed)?
Example 15
medium
Why must a balanced equation obey conservation of mass?
Example 16
medium
Limestone heated in an open crucible loses mass. Using CaCO3→CaO+CO2, explain.
Example 17
medium
Iron wool is heated in open air and gains mass. Reconcile this with conservation of mass.
Example 18
medium
100 g of reactants gives 88 g of solid product in an open dish. What mass of gas was released?
Example 19
medium
12 g of carbon burns completely in oxygen to form 44 g of CO2. What mass of oxygen reacted?
Example 20
challenge
A student burns 6 g of carbon in a sealed flask containing 16 g O2. After full reaction, what is in the flask and its total mass?
Example 21
challenge
In an open beaker, 2g of mass seems to disappear during a reaction. Argue from atoms why no mass was destroyed and identify the cause.
Example 22
challenge
Hydrogen (2 g) and oxygen (16 g) react. Only 18 g of water forms and some oxygen remains. Use conservation to find the leftover oxygen mass.
Example 23
easy
5 g of A reacts completely with 3 g of B in a sealed flask. What is the total mass of products?
Example 24
easy
True or false: total mass before a reaction can be less than after.
Example 25
easy
A balloon contains baking soda and vinegar. After they react inside, does the balloon-and-contents system change mass?
Example 26
easy
A reaction in an open flask appears to gain mass. What is the likely cause?
Example 27
easy
20 g of magnesium burns in air to form 33 g of magnesium oxide. What mass of oxygen reacted?
Example 28
medium
2H2+O2→2H2O. If 8 g H2 reacts completely with 64 g O2, what mass of water forms?
Example 29
medium
Decomposing 100 g of CaCO3 in an open crucible gives 56 g of CaO. What mass of CO2 escaped?
Example 30
medium
44 g of propane is burned completely. The products are 132 g of CO2 and 72 g of H2O. What mass of O2 was consumed?
Example 31
medium
Why does the count of atoms of each element have to match on both sides of a balanced equation?
Example 32
medium
23 g Na reacts with 35.5 g Cl2 to form NaCl. Wait — Cl2 is in excess; only 17.75 g reacts. What mass of NaCl forms?
Example 33
medium
In N2+3H2→2NH3, 14 g N2 reacts with 3 g H2 in a sealed flask. What mass of NH3 forms (assuming complete reaction)?
Example 34
medium
Why does steel wool gain mass when heated in air?
Example 35
medium
If 23 g of Na reacts with 35.5 g of Cl (atomic) to form NaCl, what mass of NaCl is produced?
Example 36
hard
A 5.6 g iron nail is left in a damp environment. After full rusting to Fe2O3, it weighs 8.0 g. What mass of oxygen was incorporated?
Example 37
hard
A student measures 200 g of reactants in a beaker and 188 g of solid product, with the beaker uncovered. Is this consistent with conservation of mass?
Example 38
hard
A reaction A+B→C+D runs. Mass A = 18 g, mass B = 32 g. Product D has mass 10 g and escapes as a gas. What mass of C remains?
Example 39
hard
A sealed bag has 25 g of reactants. After a reaction it has the same mass but feels warm. Use conservation to identify what is and is not changing.
Example 40
hard
12 g of carbon burns completely in 32 g of oxygen and forms 44 g of CO2. Show numerically that mass is conserved.
Example 41
hard
A photographer claims a reaction 'made matter disappear' because the products weigh less than the reactants. What flaw is in this claim?
Example 42
hard
2Al+3Cl2→2AlCl3. If 5.4 g Al (M = 27) reacts completely with Cl2 to form 26.7 g AlCl3, what mass of Cl2 was used?
Example 43
challenge
A flask is sealed and weighed: 250.0 g. A reaction occurs that produces a gas inside. After a few minutes the seal is broken and the flask is weighed at 248.5 g. Without the seal broken it weighed 250.0 g. Explain both readings using conservation of mass.