Unit 6: The Mole and Stoichiometry
The counting unit that lets chemists weigh out atoms and molecules on a balance, and the arithmetic that turns a balanced equation into real masses.
12 topics · 49 questions
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6.1 Why Chemists Count in Moles
- How it worksHow does grouping an enormous number of atoms into one mole make counting them practical?
- How it worksHow does grouping particles into a mole let a chemist skip counting entirely and just weigh a sample on a balance?
- Common mistakeWhy isn't a mole a unit of mass the way a gram is?
- Going furtherWhat happens to the number of particles in a sample when you compare one mole of helium to one mole of gold?
- Ties togetherHow does treating the mole as a fixed counting number set up the mass-to-mole conversions used throughout the rest of this unit?
Your turn1 of 2
A single gram of hydrogen contains roughly 600 billion trillion atoms, a number so large that counting them individually one at a time, even at a billion per second, would take far longer than the current age of the universe. Why would counting atoms one at a time take longer than the age of the universe?6.2 Avogadro's Number as a Counting Unit
- How it worksHow does 6.022 times 10 to the 23rd particles become the fixed size of one mole?
- Worked exampleFind how many water molecules are in exactly 2 moles of water.
- Common mistakeWhy doesn't Avogadro's number change depending on which substance you're counting?
- Going furtherWhat happens to the count of particles you have when you compare half a mole of atoms to half a dozen eggs?
- Ties togetherHow does Avogadro's number connect this counting idea to the molar mass calculations used later in the unit?
Your turn1 of 2
A sample contains 10.51 moles of water. Using Avogadro's number, 6.022 times 1023 particles per mole, if the number of water molecules is written as (coefficient) times 1023, what is the coefficient?6.3 Converting Between Moles and Number of Particles
- How it worksHow does Avogadro's number act as the conversion factor between moles and individual particles?
- Common mistakeWhy doesn't converting moles to particles ever change how much a sample actually weighs?
- Worked exampleFind how many atoms are in 0.5 moles of iron.
Your turn1 of 2
A sample contains 8.37 moles of iron. Using Avogadro's number, 6.022 times 1023 atoms per mole, if the number of iron atoms is written as (coefficient) times 1023, what is the coefficient?6.4 Molar Mass
- How it worksHow does adding up the atomic masses on the periodic table give you a compound's molar mass?
- Common mistakeWhy isn't molar mass the same thing as an atom's mass number?
- Worked exampleFind the molar mass of water, H2O.
Your turn1 of 2
A compound's formula contains 4 atoms of an element with atomic mass 26 grams per mole, and 6 atoms of a second element with atomic mass 23 grams per mole. What is the compound's molar mass?6.5 Converting Between Grams and Moles
- How it worksHow does molar mass act as the conversion factor between grams and moles?
- Worked exampleFind how many moles are in 36 grams of water.
- Worked exampleFind the mass in grams of 2.5 moles of carbon dioxide, CO2.
- Common mistakeWhy doesn't converting grams to moles change the identity of the substance you're measuring?
Your turn1 of 2
Find how many moles are in 349 grams of a substance with a molar mass of 163 grams per mole.6.6 Percent Composition of a Compound
- How it worksHow do you turn a compound's formula into the percent by mass of each element it contains?
- Worked exampleFind the percent composition by mass of carbon in carbon dioxide, CO2.
- Worked exampleFind the percent composition by mass of oxygen in water, H2O.
- Common mistakeWhy doesn't percent composition by mass match the percent of atoms each element contributes?
Your turn1 of 2
A compound has a total molar mass of 285 grams per mole, of which 13 grams per mole comes from one particular element. What percent of the compound's mass by mass comes from that element?6.7 Empirical Formula from Percent Composition
- How it worksHow do you convert percent composition into mole ratios and then into the smallest whole-number formula?
- Worked exampleFind the empirical formula of a compound that is 40 percent carbon, 6.7 percent hydrogen, and 53.3 percent oxygen by mass.
- Worked exampleFind the empirical formula of a compound that is 43.4 percent sodium, 11.3 percent carbon, and 45.3 percent oxygen by mass.
- Common mistakeWhy isn't an empirical formula always the same as the compound's true molecular formula?
Your turn1 of 2
A sample of a compound weighs 212 grams total, and 60 percent of that mass by mass comes from one particular element. How many grams of that element are present in the sample?6.8 Molecular Formula from Empirical Formula
- How it worksHow does dividing a compound's actual molar mass by its empirical formula mass reveal the true molecular formula?
- Worked exampleFind the molecular formula of a compound with empirical formula CH2O and a molar mass of 180 g/mol.
- Common mistakeWhy doesn't the empirical formula alone tell you how many atoms are actually in one molecule?
- Ties togetherHow does finding a compound's molecular formula from its empirical formula depend on already knowing its molar mass?
Your turn1 of 2
A compound's empirical formula has a mass of 46 grams per mole, and the compound's actual molar mass is 250 grams per mole. By what factor must the empirical formula's subscripts be multiplied to get the molecular formula?6.9 Mole Ratios from a Balanced Equation
- How it worksHow do the coefficients in a balanced equation set the exact mole ratio between any two substances in it?
- Worked exampleFind the mole ratio of hydrogen to nitrogen in the balanced equation N2 + 3H2 makes 2NH3.
- Common mistakeWhy doesn't a mole ratio depend on how many grams of each substance you actually have?
- Ties togetherHow does balancing a chemical equation in the first place set up the exact mole ratios you read off it here?
Your turn1 of 2
In the balanced equation N2 + 3H2 makes 2NH3, the coefficients set a fixed 1 to 3 mole ratio between N2 and H2. If a reaction uses 20.95 moles of N2, how many moles of H2 does it require?6.10 Stoichiometry: Mass-to-Mass Calculations
- How it worksHow does converting grams to moles, applying the mole ratio, and converting back to grams solve a stoichiometry problem?
- Worked exampleFind the mass of water produced when 8 grams of hydrogen gas reacts completely with excess oxygen in 2H2 + O2 makes 2H2O.
- Worked exampleFind the mass of carbon dioxide produced when 10 grams of propane burns completely in C3H8 + 5O2 makes 3CO2 + 4H2O.
- Common mistakeWhy can't you skip straight from grams of one substance to grams of another without passing through moles?
Your turn1 of 2
In the reaction 2H2 + O2 makes 2H2O, 42 grams of hydrogen gas (molar mass 2.016 g/mol) reacts completely with excess oxygen. Using the 1 to 1 mole ratio between H2 and H2O, and water's molar mass of 18.02 g/mol, how many grams of water are produced?6.11 The Limiting Reactant
- How it worksHow does the reactant that runs out first cap the amount of product a reaction can make?
- Worked exampleFind the limiting reactant when 4 moles of H2 react with 1 mole of N2 in N2 + 3H2 makes 2NH3.
- Worked exampleFind the limiting reactant when 20 grams of magnesium reacts with 20 grams of oxygen gas in 2Mg + O2 makes 2MgO.
- Common mistakeWhy isn't the limiting reactant simply whichever substance you started with the smallest mass of?
- Going furtherWhat happens to the leftover excess reactant if you double the amount of limiting reactant you start with?
Your turn1 of 2
In the reaction N2 + 3H2 makes 2NH3, a reaction starts with 27.95 moles of H2 and plenty of excess N2. Since H2 is the limiting reactant here, and the mole ratio between H2 and NH3 is 3 to 2, how many moles of NH3 can form once all the H2 has reacted?6.12 Theoretical Yield and Percent Yield
- How it worksHow does dividing actual yield by theoretical yield turn a lab result into a percent yield?
- Worked exampleFind the percent yield if a reaction's theoretical yield is 50 grams and the actual yield collected is 42 grams.
- Common mistakeWhy doesn't a percent yield over 100 percent mean a reaction somehow created extra matter?
- Ties togetherHow does a limiting reactant calculation set the theoretical yield that percent yield gets measured against?
Your turn1 of 2
A reaction's calculated theoretical yield is 282 grams, but the actual yield collected in the lab after the reaction is 248 grams. What is the percent yield?