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"25 Stoichiometry Practice Problems" by SciGirl153

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"6.4 Stoichiometry" by Lake Washington Institute of Technology

Includes written explanations, examples, and practice problems.

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Consider a classic recipe for pound cake: 1 pound of eggs, 1 pound of butter, 1 pound of flour, and 1 pound of sugar. (That's why it's called "pound cake.") If you have 4 pounds of butter, how many pounds of sugar, flour, and eggs do you need? You would need 4 pounds each of sugar, flour, and eggs.

Now suppose you have 1.00 g H ₂. If the chemical reaction follows the balanced chemical equation 2H ₂(g) + O ₂(g) → 2H ₂ O(ℓ), then what mass of oxygen do you need to make water?

Curiously, this chemical reaction question is very similar to the pound cake question. Both of them involve relating a quantity of one substance to a quantity of another substance or substances. The relating of one chemical substance to another using a balanced chemical reaction is called stoichiometry. Using stoichiometry is a fundamental skill in chemistry; it greatly broadens your ability to predict what will occur and, more importantly, how much is produced.

Let us consider a more complicated example. A recipe for pancakes calls for 2 cups (c) of pancake mix, 1 egg, and ½ c of milk. We can write this in the form of a chemical equation:

2 c mix + 1 egg + ½ c milk → 1 batch of pancakes

If you have 9 c of pancake mix, how many eggs and how much milk do you need? It might take a little bit of work, but eventually you will find you need 4½ eggs and 2¼ c milk.

How can we formalize this? We can make a conversion factor using our original recipe and use that conversion factor to convert from a quantity of one substance to a quantity of another substance. Because one recipe's worth of pancakes requires 2 c of pancake mix, 1 egg, and ½ c of milk, we actually have the following mathematical relationships that relate these quantities:

2 c pancake mix ⇔ 1 egg ⇔ ½ c milk

where ⇔ is the mathematical symbol for "is equivalent to." This does not mean that 2 c of pancake mix equals 1 egg. However, as far as this recipe is concerned, these are the equivalent quantities needed for a single recipe of pancakes. So, any possible quantities of two or more ingredients must have the same numerical ratio as the ratios in the equivalence.

We can deal with these equivalences in the same way we deal with equalities in unit conversions: we can make conversion factors that essentially equal 1. For example, to determine how many eggs we need for 9 c of pancake mix, we construct the conversion factor:

[latex]\dfrac{1\text{ egg}}{2\text{ c pancake mix}}[/latex]

This conversion factor is, in a strange way, equivalent to 1 because the recipe relates the two quantities. Starting with our initial quantity and multiplying by our conversion factor, we get:

Note how the units cups pancake mix cancelled, leaving us with units of eggs. This is the formal, mathematical way of getting our amounts to mix with 9 c of pancake mix. We can use a similar conversion factor for the amount of milk:

Again, units cancel, and new units are introduced.

A balanced chemical equation is nothing more than a recipe for a chemical reaction. The difference is that a balanced chemical equation is written in terms of atoms and molecules, not cups, pounds, and eggs.

For example, consider the following chemical equation:

2H ₂(g) + O ₂(g) → 2H ₂ O(ℓ)

We can interpret this as, literally, "two hydrogen molecules react with one oxygen molecule to make two water molecules." That interpretation leads us directly to some equivalences, just as our pancake recipe did:

2 H ₂ molecules ⇔ 1 O ₂ molecule ⇔ 2 H ₂ O molecules

These equivalences allow us to construct conversion factors:

[latex]\dfrac{2\text{ molecules }\ce{H2}}{1\text{ molecule }\ce{O2}}\Leftrightarrow \dfrac{2\text{ molecules }\ce{H2}}{2\text{ molecules }\ce{H2O}} \Leftrightarrow \dfrac{1\text{ molecule }\ce{O2}}{2\text{ molecules }\ce{H2O}}[/latex]

and so forth. These conversions can be used to relate quantities of one substance to quantities of another. For example, suppose we need to know how many molecules of oxygen are needed to react with 16 molecules of H ₂. As we did with converting units, we start with our given quantity and use the appropriate conversion factor:

[latex]16\text{ }\cancel{\text{molecules }\ce{H2}}\times \dfrac{1\text{ molecule }\ce{O2}}{2\text{ }\cancel{\text{molecules }\ce{H2}}}=8\text{ molecules }\ce{O2}[/latex]

Note how the unit molecules H ₂ cancels algebraically, just as any unit does in a conversion like this. The conversion factor came directly from the coefficients in the balanced chemical equation. This is another reason why a properly balanced chemical equation is important.

87 molecules

Chemical equations also allow us to make conversions regarding the number of atoms in a chemical reaction because a chemical formula lists the number of atoms of each element in a compound. The formula H 2 O indicates that there are two hydrogen atoms and one oxygen atom in each molecule, and these relationships can be used to make conversion factors:

[latex]\dfrac{2\text{ atoms H}}{1\text{ molecule }\ce{H2O}} \Leftrightarrow \dfrac{1\text{ molecule }\ce{H2O}}{1\text{ atom O}}[/latex]

Conversion factors like this can also be used in stoichiometry calculations.

259 molecules

For a video lecture on stoichiometry, view this video on stoichiometry by Dr. Jessie A. Key.

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Consider a classic recipe for pound cake: 1 pound of eggs, 1 pound of butter, 1 pound of flour, and 1 pound of sugar. (That's why it's called "pound cake.") If you have 4 pounds of butter, how many pounds of sugar, flour, and eggs do you need?

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"5.2.1: Practice Problems - Reaction Stoichiometry" by LibreTexts

A set of various practice problems with answers. Answers are hidden so you can't see the answer right away.

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PROBLEM \(\PageIndex{1}\)

Write the balanced equation and determine the information requested. Don't worry about state symbols in these reactions.

1. The number of moles and the mass (in grams) of chlorine, Cl₂, required to react with 10.0 g of sodium metal, Na, to produce sodium chloride, NaCl.

2. The number of moles and the mass (in milligrams) of diatomic oxygen formed by the decomposition of 1.252 g of mercury(II) oxide.

3. The number of moles and the mass (in g) of sodium nitrate, NaNO₃, required to decompose and produce 128 g of diatomic oxygen, where NaNO₂ is the other product.

4. The number of moles and the mass (in kg) of carbon dioxide formed by the combustion of 20.0 kg of carbon in an excess of diatomic oxygen.

5. The number of moles and the mass (in kg) of copper(II) carbonate needed to decompose in order to produce 1.500 kg of copper(II) oxide, where CO₂ is the other product.

6. The number of moles and mass (in grams) of C₂H₄ required to react with water to produce 9.55 g C₂H₆O.

Click here to see a video of the solution

*Apologies for the brief phone ringing*

PROBLEM \(\PageIndex{2}\)

I ₂ is produced by the reaction of 0.4235 mol of CuCl ₂ according to the following equation: \(\ce{2CuCl2 + 4KI \rightarrow 2CuI + 4KCl + I2}\) .

PROBLEM \(\PageIndex{3}\)

Silver is often extracted from ores as K[Ag(CN) ₂] and then recovered by the reaction

\(\ce{2K[Ag(CN)2]}(aq)+\ce{Zn}(s)\rightarrow \ce{2Ag}(s)+\ce{Zn(CN)2}(aq)+\ce{2KCN}(aq)\)

PROBLEM \(\PageIndex{4}\)

What mass of silver oxide, Ag ₂O, is required to produce 25.0 g of silver sulfadiazine, AgC ₁₀H ₉N ₄SO ₂, from the reaction of silver oxide and sulfadiazine?

\(\ce{2C10H10N4SO2 + Ag2O \rightarrow 2AgC10H9N4SO2 + H2O}\)

PROBLEM \(\PageIndex{5}\)

Carborundum is silicon carbide, SiC, a very hard material used as an abrasive on sandpaper and in other applications. It is prepared by the reaction of pure sand, SiO ₂, with carbon at high temperature. Carbon monoxide, CO, is the other product of this reaction. Write the balanced equation for the reaction, and calculate how much SiO ₂ is required to produce 3.00 kg of SiC.

PROBLEM \(\PageIndex{6}\)

Automotive air bags inflate when a sample of sodium azide, NaN ₃, is very rapidly decomposed.

\(\ce{2NaN3}(s) \rightarrow \ce{2Na}(s) + \ce{3N2}(g)\)

What mass of sodium azide is required to produce 2.6 ft ³ (73.6 L) of nitrogen gas with a density of 1.25 g/L?

PROBLEM \(\PageIndex{7}\)

Urea, CO(NH ₂) ₂, is manufactured on a large scale for use in producing urea-formaldehyde plastics and as a fertilizer. What is the maximum mass of urea that can be manufactured from the CO ₂ produced by combustion of 1.00×10 ³ kg of carbon followed by the reaction?

\[\ce{CO2}(g)+\ce{2NH3}(g)\rightarrow \ce{CO(NH2)2}(s)+\ce{H2O}(l)\]

PROBLEM \(\PageIndex{8}\)

In an accident, a solution containing 2.5 kg of nitric acid was spilled. Two kilograms of Na ₂CO ₃ was quickly spread on the area and CO ₂ was released by the reaction. Was sufficient Na ₂CO ₃ used to neutralize all of the acid? (in this reaction, water and sodium nitrate are the other two products)

Answer

No, you will need 2.1 kg of sodium carbonate to neutralize 2.5 kg of nitric acid.

PROBLEM \(\PageIndex{9}\)

A compact car gets 37.5 miles per gallon on the highway. If gasoline contains 84.2% carbon by mass and has a density of 0.8205 g/mL, determine the mass of carbon dioxide produced during a 500-mile trip (3.785 liters per gallon).

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5.2.1: Practice Problems- Reaction Stoichiometry

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PROBLEM \(\PageIndex{1}\) Write the balanced equation and determine the information requested. Don't worry about state symbols in these reactions. The number of moles and the mass (in grams) of chlorine, Cl2, required to react with 10.0 g of sodium metal, Na, to produce sodium chloride, NaCl.

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"Stoichiometry Tutorial: Step by Step Video + review problems explained" by Crash Course Academy on YouTube

Watch this video for some extra help and explanations with visuals!

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Stoichiometry Tutorial: Step by Step Video + review problems explained | Crash Chemistry Academy

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"Stoichiometry Made Easy: Stoichiometry Tutorial Part 1" by ketzbook on YouTube

Watch this video to better understand stoich!

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Stoichiometry Made Easy: Stoichiometry Tutorial Part 1

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"Stoichiometry" by Khan Academy

Lots of helpful resources!

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Introduction

A balanced chemical equation is analogous to a recipe for chocolate chip cookies. It shows what reactants (the ingredients) combine to form what products (the cookies). It also shows the numerical relationships between the reactants and products (such as how many cups of flour are required to make a single batch of cookies).

These numerical relationships are known as reaction stoichiometry, a term derived from the Ancient Greek words stoicheion ("element") and metron ("measure"). In this article, we'll look at how we can use the stoichiometric relationships contained in balanced chemical equations to determine amounts of substances consumed and produced in chemical reactions.

Balanced equations and mole ratios

A common type of stoichiometric relationship is the mole ratio, which relates the amounts in moles of any two substances in a chemical reaction. We can write a mole ratio for a pair of substances by looking at the coefficients in front of each species in the balanced chemical equation. For example, consider the equation for the reaction between iron(III) oxide and aluminum metal:

Example: Using mole ratios to calculate mass of a reactant

Consider the following unbalanced equation:

Step 1: Convert known reactant mass to moles

Step 2: Use the mole ratio to find moles of other reactant

Step 3: Convert moles of other reactant to mass

Shortcut: We could have combined all three steps into a single calculation, as shown in the following expression:

Be sure to pay extra close attention to the units if you take this approach, though!

A balanced chemical equation shows us the numerical relationships between each of the species involved in the chemical change. We can use these numerical relationships to write mole ratios, which allow us to convert between amounts of reactants and/or products (and thus solve stoichiometry problems!).

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Stoichiometry (article) | Chemical reactions | Khan Academy

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Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.

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Stoich. Video Involving Atoms, Molecules, and Ions

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Stoich. Video Involving Atoms, Molecules, and Ions

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"Stoichiometry for Atoms, Molecules, and Ions" by Osmosis from Elsevier.

Helpful video for conversions other than just grams!

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"Stoichiometry Tutorials: Using Molecular Weight" by The ChemCollective

Helpful video for help with molecular weight/molar mass!

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(from a complete OLI stoichiometry course)

A previous tutorial shows how to calculate the molecular weight of a substance from the atomic weights given. On this page, we use the molecular weight to convert between the macroscopic scale (grams of a substance) and the microscopic scale (number of molecules of that substance).

Using Molecular Weight Movie Text

Molecular weight is an important tool in stoichiometry, because it allows us to connect between the macroscopic scale, which is what we percieve in our everyday lives, to the molecular scale, where chemistry occurs. On the macroscopic scale, we think in terms of grams of a substance, while on the molecular scale, we think in terms of numbers of molecules, in moles.

To connect between the macroscopic and molecular scale, we use the molecular weight. For example, the molecular weight of water is 18.015 grams per mole. This gives us the two ratios shown in the image below, which we can use to convert between grams and moles.

We can use this to get from a quantity expressed in grams like 7.4 grams of water, a macroscopic quantity that we might weigh out in a lab, to the quantity expressed in number of moles of water, a molecular quantity. To do this, we take our existing 7.4 grams water and multiply by our ratio of 1 mole of water per 18.015 grams of water. Remember, this ratio is equal to one, so multiplying by it converts the units without making any difference to the actual quantity of water. Next, we do the math and get a value 0.41. The units of grams of water cancel, so the units of the result are moles of water.

The content above has been converted from Adobe Flash Player and may not display correctly.

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This tutorial explains use the molecular weight to convert between the macroscopic scale (grams of a substance) and the microscopic scale (number of molecules of that substance). Guided practice performing molecular weight calculations is provided.

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"Introduction to Balancing Chemical Equations" by Tyler DeWitt

Watch this video if you need help with balancing!

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Introduction to Balancing Chemical Equations

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Stoichiometry Scavenger Hunt Practice Problems

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Stoichiometry Scavenger Hunt Practice Problems

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"Stoichiometry Scavenger Hunt" by Nora Walsh on Chem Ed Xchange.

See if you can complete the problems in the correct order! The answer will tell you the next clue to solve!

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