⏱️ October 22, 2020
In the previous section, we looked at qualitatively analyzing chemical reactions. Now, we’ll learn how to quantitatively analyze reactions using stoichiometry! It may seem like a lot of math at first, but once you do more practice, you’ll be more confident and become a stoichiometry master 👨🏫
In earlier units, you learned about moles, molar mass/molar volume, molarity, and Avogadro's number. Stoichiometry is all about using mole ratios and these measurements and manipulating them to get to our desired unit. For this reason, it’s important to comfortably know the relationship between these ideas as we do mole ratios. Below is a list of these measurements: 1 mole = molar volume (gas) (At STP, this is 22.4 L/mol. That number is also on your AP equation sheet)
1 mole = 6.022 x 10^23 particles (This is Avogrado’s number, which is on your AP equation sheet!)
1 mole = molar mass (Each element’s is on the periodic table. Hydrogen’s is 1.008 g/mol) You can find mole ratios by looking at a chemical reaction🧪. The coefficients of the reaction represent the ratio of molecules necessary in order for the reaction to complete. In the reaction below, 1 mole of C2H5OH (ethanol) and 1 mole of oxygen gas are needed to produce 2 moles of carbon dioxide and 3 moles of water. A sample mole ratio would be 1 mole of O2 to 2 moles of CO2. Keep in mind that 2 moles of CO2 to 1 mole of O2 would mean the same thing.
How many moles of potassium are required to fully react with 11.6 moles of water?
Write the chemical reaction, if it’s not given. Be sure to balance it. 2K (s) + 2H2O (l) --> 2KOH (aq) + H2 (g)
Identify the known measurement. In this case, we’re given 11.6 moles of water💧.
Since our given value is already in moles, we just need to write the appropriate mole ratio. We want a ratio that cancels❌ out the known measurement units (moles of H2O) and brings in the unit we want (moles of K).
Looking at the chemical equation, for every 2 moles of H2O, we need 2 moles of K.
💡Note: Writing out the units and the molecule the value belongs to will help you exponentially. Please do it!
One way to make sure you’re writing the right mole track of the units is to cross out whatever cancels out and circle the unit you’re left with, like so:
If you have 105.2 g of ethanol (C2H5OH), what is the maximum volume of carbon dioxide that can form at STP?
Write the chemical equation and balance it.
C2H5OH (aq) + O2 (g) --> 2CO2 (g) + 3H2O (g)
Identify the known measurement: 105.2 g of ethanol.
Since our given value is in grams, we need to convert it to moles. To do this, we will calculate the molar mass (g/mol) of C2H5OH.
2(12.01g) + 6(1.008g) + 16.00g = 46.07 g/mol
Using this value, we can make a mole ratio.
105.2 g of C2H5OH * 1 mole C2H5OH/46.06 g of C2H5OH
Uh oh!😩 We don’t want moles of C2H5OH--we want the volume of carbon dioxide. Now that we have moles, though, we can go over to CO2 with a mole ratio. Looking at the equation, for every 1 mole of C2H5OH, we can make up to 2 moles of CO2.
Almost there🎉! We have a unit of CO2, but we want it’s volume. Since this reaction occurs at STP, we can use the value of the molar volume at STP.
Good work! With 105.2g of ethanol, we could make 102. L of CO2😮.
You will see stoichiometry used with the ideal gas law (PV=nRT) and molarity (moles/mass or moles/volume, often denoted as M). Check out the practice problems to see how you’ll see it in problems.
Here is a neat list of the general steps for tackling stoichiometry problems:
Write out the balanced chemical equation, if not given
Identify the known measurements given to you
If the known measurement is in grams, convert it to moles
Multiply by a mole ratio using the coefficients in the chemical equation.
Repeat steps 3-4 as many times as necessary.
Cross out your units and circle the final one to make sure you set it up correctly!
The following reaction occurs at STP:
How many particles of BrF will be produced with 160.0g of Br2?
Step 1 includes using the molar mass of Br2. Step 2 is the mole ratio of Br2 to BrF. Step 3 uses Avogadro's number to convert to the number of particles of BrF.
🎥 Watch: AP Chemistry - Stoichiometry (Part 2)
✍️ Free Response Questions
AP Chemistry Free Response Questions
⚛️ Unit 1: Atomic Structure and Properties
1.1Moles and Molar Mass
1.2Mass Spectroscopy of Elements
1.3Elemental Composition of Pure Substances
1.4Composition of Mixtures
1.5Atomic Structure and Electron Configurations
1.6Photoelectron Spectroscopy & Graph Interp.
🤓 Unit 2: Molecular and Ionic Compound Structures and Properties
2.0Unit 2 Overview: Molecular and Ionic Bonding
2.1Types of Chemical Bonds
2.2Intramolecular Force and Potential Energy
2.3Ionic Bonding and Ionic Solids
2.4Metallic Bonding and Alloys
2.5Lewis Dot Diagrams
2.6Resonance and Formal Charge
🌀 Unit 3: Intermolecular Forces and Properties
3.0Unit 3 Overview: Intermolecular Forces and Properties
3.2Properties of Solids
3.3Solids, Liquids, and Gases
3.4The Ideal Gas Law
3.5The Kinetic Molecular Theory of Gases
3.6Deviations from the Ideal Gas Law
3.7Mixtures and Solutions
3.8Representations of Solutions
3.9Separation of Solids/Mixtures
3.10Solubility and Solubility Rules
3.11Spectroscopy and the Electromagnetic Spectrum
3.12Quantum Mechanics and the Photoelectric Effect
🧪 Unit 4: Chemical Reactions
4.0Unit 4 Overview: Chemical Reactions
4.1Recognizing Chemical Reactions
4.2Net Ionic Equations
4.4Physical vs. Chemical Changes
4.5Stoichiometry & Calculations
4.6Titrations - Intro and Calculations
4.8Intro to Acid-Base Neutralization Reactions
👟 Unit 5: Kinetics
5.0Unit 5 Overview: Kinetics
5.1Defining Rate of Reaction
5.2Introduction to Rate Laws
5.3Rate and Concentration Change
5.4Writing Rate Laws
5.5Collision Model of Kinetics
5.6Reaction Energy and Graphs w/ Energy
5.7Reaction Mechanisms and Elementary Steps
5.8Writing Rate Laws Using Mechanisms
🔥 Unit 6: Thermodynamics
6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics
6.1Endothermic Processes vs. Exothermic Processes
6.2Energy Diagrams of Reactions
6.3Kinetic Energy, Heat Transfer, and Thermal Equilibrium
6.4Heat Capacity and Coffee-Cup Calorimetry
6.5Phase Changes and Energy
6.6Introduction to Enthalpy of Reaction
6.7Bond Enthalpy and Bond Dissociation Energy
6.8Enthalpies of Formation
⚖️ Unit 7: Equilibrium
🍊 Unit 8: Acids and Bases
8.0Unit 8 Overview: Acids and Bases
8.1Introduction to Acids and Bases
Unit 9: Applications of Thermodynamics
🤺 AP Chemistry Essentials
🧐 Multiple Choice Questions
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