Ideal Gas Law and Gas Properties - Complete Interactive Lesson
Part 1: Gas Properties & Pressure
🌬️ Gas Properties & Kinetic Molecular Theory
Part 1 of 7 — Understanding Gas Behavior
Topics in This Part
Section
💨 The Four Gas Variables
Pressure
Temperature
Volume
Amount
🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.
What You'll Master in Part 1
Understanding the core concepts covered in Part 1
Applying these ideas to solve practice problems
Building toward AP exam readiness for this topic
💨 The Four Gas Variables
Every gas sample is described by four measurable quantities:
Variable
Symbol
SI Unit
Common Units
Pressure
P
Pascal (Pa)
atm, mmHg, torr, kPa
Volume
V
m³
liters (L)
Temperature
T
Kelvin (K)
°C (must convert!)
⏱️ Kinetic Molecular Theory (KMT)
The kinetic molecular theory describes an ideal gas with the following assumptions:
Gas particles are in constant, random motion — they travel in straight lines until they collide.
Collisions are perfectly elastic — no kinetic energy is lost during collisions.
Gas particles have negligible volume — the volume of individual molecules is tiny compared to the container.
No intermolecular forces — gas particles don't attract or repel each other.
Average kinetic energy is proportional to temperature (in Kelvin):
KEavg
Gas Properties Quiz 🎯
Unit Conversion Practice 🧮
Convert the following gas measurements:
1) Convert 2.50 atm to mmHg.
2) Convert 350 K to °C (to 3 significant figures).
3) Convert 0.500 L to mL.
KMT Concepts Check 🔍
Exit Quiz — Gas Properties & KMT ✅
Part 2: Boyle's, Charles's & Avogadro's Laws
📏 Boyle's, Charles's, and Avogadro's Laws
Part 2 of 7 — The Foundational Gas Laws
Topics in This Part
Section
📏 Boyle's Law (Pressure–Volume)
Example
Conceptual Picture
📏 Charles's Law (Volume–Temperature)
Example
🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.
What You'll Master in Part 2
Understanding the core concepts covered in Part 2
Applying these ideas to solve practice problems
Building toward AP exam readiness for this topic
📏 Boyle's Law (Pressure–Volume)
At constant temperature and amount of gas:
Part 3: The Ideal Gas Law (PV=nRT)
⚗️ The Ideal Gas Law
Part 3 of 7 — PV = nRT
Topics in This Part
Section
📌 The Equation and R
The Gas Constant R
Rearranged Forms
🧪 Worked Examples
Example 1: Find Volume
🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.
What You'll Master in Part 3
Understanding the core concepts covered in Part 3
Applying these ideas to solve practice problems
Building toward AP exam readiness for this topic
📌 The Equation and R
P
Part 4: Gas Stoichiometry
⚖️ Molar Mass & Gas Density
Part 4 of 7 — Connecting Gases to Molar Mass
Topics in This Part
Section
💨 Molar Mass from Gas Data
Example
Lab Application: Dumas Method
💨 Gas Density
Key Observations
🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.
What You'll Master in Part 4
Understanding the core concepts covered in Part 4
Applying these ideas to solve practice problems
Building toward AP exam readiness for this topic
💨 Molar Mass from Gas Data
Starting with PV=nRT and substituting (where = mass, = molar mass):
Part 5: Dalton's Law of Partial Pressures
🎈 Dalton's Law of Partial Pressures
Part 5 of 7 — Gas Mixtures
Topics in This Part
Section
📏 Dalton's Law
Using the Ideal Gas Law
Example
⚖️ Mole Fraction
Example
🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.
What You'll Master in Part 5
Understanding the core concepts covered in Part 5
Applying these ideas to solve practice problems
Building toward AP exam readiness for this topic
📏 Dalton's Law
The total pressure of a gas mixture equals the sum of the partial pressures of each component gas:
Part 6: Problem-Solving Workshop
🧪 Problem-Solving Workshop
Part 6 of 7 — Mixed Gas Law Calculations
Practice Makes Perfect
This workshop features multi-step problems that mirror the AP Chemistry exam format. Each problem requires you to combine concepts from previous parts and show your work clearly.
🔑 Why this matters: The AP Chemistry exam rewards students who can apply concepts to unfamiliar problems — structured practice is the best preparation.
What You'll Master in Part 6
Working through complete multi-step problems from start to finish
Building problem-solving strategies you can apply on the AP exam
Identifying which concepts to apply and in what order
🛠️ Problem-Solving Strategy
Identify what you know and what you need to find.
Choose the right law:
One gas, two sets of conditions → Combined gas law:
Part 7: Synthesis & AP Review
🏆 Synthesis & AP Review
Part 7 of 7 — Ideal vs. Real Gases & AP Exam Preparation
Bringing It All Together
This comprehensive review connects every concept from Parts 1–6 with AP-style problems. The questions are designed to mirror what you'll see on the actual exam — multi-step, multi-concept, and requiring clear written explanations.
🔑 Why this matters: AP Chemistry exam questions rarely test one concept in isolation — success requires connecting ideas across topics.
What You'll Master in Part 7
Solving AP-style questions that integrate multiple concepts from this unit
Writing clear, concise explanations using proper chemistry terminology
Identifying and avoiding common AP exam traps and mistakes
📌 Ideal vs. Real Gases
⚠️ Warning: The ideal gas law is an approximation! Real gases deviate from ideal behavior, especially at high pressures and low temperatures.
The ideal gas law works well under many conditions, but real gases deviate from ideal behavior when:
Pressure is force per unit area: P=F/A. Gas molecules exert pressure by colliding with container walls.
Key conversions:
1 atm=760 mmHg=760 torr=101.325 kPa
Temperature
Temperature must always be in Kelvin for gas law calculations:
T(K)=T(°C)+273.15
⚠️ Warning: Always convert to Kelvin before any gas law calculation. Using Celsius will give incorrect results!
At 0 K (absolute zero), molecular motion theoretically stops.
Volume
Volume is typically measured in liters (L) in chemistry. 1 L=1000 mL=0.001 m3.
Amount
The amount of gas is measured in moles (n), which connects to the number of particles via Avogadro's number (6.022×1023).
=
23
kB
T
where kB=1.38×10−23 J/K is the Boltzmann constant.
🔑 Key Concept: Average kinetic energy depends only on temperature — not on the identity or mass of the gas.
Root Mean Square Speed
The average speed of gas molecules depends on temperature and molar mass:
vrms=M3RT
where R=8.314 J/(mol·K) and M is the molar mass in kg/mol.
Key insight: Lighter molecules move faster at the same temperature.
P1V1=P2V2
Pressure and volume are inversely proportional. When you compress a gas (decrease volume), the pressure increases — and vice versa.
Example
Problem: A gas occupies 4.00 L at 1.00 atm. What is the volume if the pressure increases to 2.00 atm?
Solution:
V2=P2P1V1=2.00(1.00)(4.00)=2.00 L
Conceptual Picture
When volume decreases, molecules hit the walls more often → more collisions per second → higher pressure.
📏 Charles's Law (Volume–Temperature)
At constant pressure and amount of gas:
T1V1=T2V2
Volume and temperature are directly proportional (temperature in Kelvin!). Heat a gas and it expands; cool it and it contracts.
Example
Problem: A balloon has a volume of 2.50 L at 20°C. What volume will it have at 80°C? (Pressure constant)
Solution:
T1=20+273.15=293.15 K,T
V2=V1×
Why Kelvin?
⚠️ Warning: If you use Celsius, 0°C would imply zero volume — which is nonsensical. Kelvin starts at absolute zero, where molecular motion stops.
📏 Avogadro's Law (Volume–Amount)
At constant temperature and pressure:
n1V1=n2V2
Volume and amount of gas (in moles) are directly proportional. More molecules → more volume (at the same T and P).
Example
Problem: 3.00 mol of gas occupies 6.00 L. What volume will 5.00 mol occupy under the same conditions?
Solution:
V2=V1×
At STP
💡 Tip: At standard temperature and pressure (STP: 0°C, 1 atm), 1 mole of any ideal gas occupies 22.4 L. This is the molar volume at STP — a useful shortcut!
Gas Law Identification Quiz 🎯
Gas Law Calculations 🧮
1) A gas at 3.00 atm occupies 12.0 L. What is the volume at 1.50 atm? (constant T and n, in L)
2) A gas occupies 500 mL at 27°C. What volume will it occupy at 127°C at constant pressure? (in mL, round to nearest whole number)
3) 4.00 mol of gas occupies 10.0 L at a given T and P. How many moles would occupy 25.0 L? (in mol, to 3 significant figures)
Match the Relationship 🔍
Exit Quiz — Gas Laws ✅
V
=
n
RT
Variable
Meaning
Units
P
Pressure
atm (or kPa)
V
Volume
L
n
Amount
mol
R
Gas constant
depends on units
T
Temperature
K (always!)
The Gas Constant R
The value of R depends on your pressure unit:
R value
When to use
0.0821 L·atm/(mol·K)
When P is in atm
8.314 L·kPa/(mol·K)
When P is in kPa
8.314 J/(mol·K)
For energy calculations
🔑 Key Concept: Most AP Chemistry problems use R=0.0821 L·atm/(mol·K). Match your R value to the pressure unit!
Rearranged Forms
Solve for V: V=PnRT
Solve for P: P=VnRT
Solve for n: n=RTPV
Solve for T: T=nRPV
🧪 Worked Examples
Example 1: Find Volume
Problem: What volume does 0.500 mol of gas occupy at 1.20 atm and 25°C?
Solution:
T=25+273.15=298.15 K
V=PnRT=1.20(0.500)(0.0821)(298.15)
Example 2: Find Pressure
Problem: 2.00 mol of gas is in a 15.0 L container at 300 K. What is the pressure?
Solution:
P=VnRT=
Example 3: Find Moles
Problem: A gas at 2.50 atm and 350 K occupies 5.00 L. How many moles?
Solution:
n=RTPV=
Unit Conversion Reminder
⚠️ Warning: Always check your units before plugging in:
P in atm (if using R=0.0821)
V in liters
T in Kelvin
n in moles
Ideal Gas Law Concept Check 🎯
PV = nRT Calculations 🧮
Use R=0.0821 L·atm/(mol·K). Round to appropriate significant figures.
1) What volume (in L) does 1.00 mol of ideal gas occupy at STP (0°C, 1.00 atm)? (to 3 significant figures)
2) What pressure (in atm) is exerted by 3.50 mol of gas in a 20.0 L container at 400 K? (to 3 significant figures)
3) How many moles of gas are in a 10.0 L container at 2.00 atm and 27°C? (to 3 significant figures)
Quick Concept Sort 🔍
Exit Quiz — The Ideal Gas Law ✅
n=m/M
m
M
PV=MmRT
Solving for molar mass:
M=PVmRT
Example
Problem: A 0.325 g sample of gas occupies 225 mL at 100°C and 0.960 atm. Find the molar mass.
This matches ethanol (C₂H₅OH), which has M=46.07 g/mol.
Lab Application: Dumas Method
💡 Tip: The Dumas method is a classic lab technique for finding the molar mass of volatile liquids.
In the Dumas method for finding molar mass:
Vaporize a liquid in a flask of known volume
Measure the temperature and atmospheric pressure
Condense the vapor and weigh it
Use M=mRT/(PV)
💨 Gas Density
Density is mass per volume: d=m/V. From PV=(m/M)RT:
Vm=RTPM
d=RTPM
Key Observations
Gas density increases with pressure (more molecules per volume)
Gas density decreases with temperature (gas expands)
Gas density increases with molar mass (heavier molecules)
Example
Problem: What is the density of O₂ (M=32.00 g/mol) at STP?
Solution:
d=RTPM=
Comparing Gases
At the same T and P, the ratio of gas densities equals the ratio of molar masses:
d2d1=
Molar Mass & Density Concepts 🎯
Molar Mass & Density Calculations 🧮
Use R=0.0821 L·atm/(mol·K).
1) A 1.56 g sample of gas occupies 1.00 L at 27°C and 1.00 atm. What is the molar mass? (in g/mol, to 3 significant figures)
2) What is the density of N₂ (M=28.02 g/mol) at 25°C and 1.00 atm? (in g/L, to 3 significant figures)
3) A gas has a density of 3.17 g/L at STP. What is its molar mass? (in g/mol, to 3 significant figures)
Density Relationships 🔍
Exit Quiz — Molar Mass & Density ✅
Ptotal=P1+P2+P3+⋯
Each partial pressure is the pressure that gas would exert if it alone occupied the entire container.
Using the Ideal Gas Law
Since PV=nRT, each gas independently obeys:
Pi=VniRT
And the total:
Ptotal=VntotalRT
Example
Problem: A 10.0 L container at 300 K holds 0.200 mol N₂ and 0.300 mol O₂.
Solution:
PN2=10.0(0.200)(0.0821)(300)=0.493 atm
PO2=10.0(0.300)(0.0821)(300)=0.739 atm
Ptotal=0.493+0.739=1.232 atm
⚖️ Mole Fraction
The mole fraction (χ) of a component is the fraction of total moles that it contributes:
χi=ntotalni
The partial pressure is related to mole fraction by:
Pi=χi×P
Example
Problem: A mixture has 2.0 mol He and 3.0 mol Ne at a total pressure of 5.0 atm.
Solution:
χHe=2.0+3.02.0=0.40
PHe=0.40×5.0=2.0 atm
χNe=5.03.0
Note: All mole fractions must add up to 1.0: χHe+χNe=0.40+0.60 ✓
💨 Gas Collection Over Water
💡 Tip: When a gas is collected by displacement of water, the collected gas is mixed with water vapor. You must subtract the vapor pressure of water:
Pgas=Ptotal−PH2O
The vapor pressure of water depends on temperature (values are given in data tables).
Example
Oxygen is collected over water at 25°C. The total pressure is 752 mmHg. The vapor pressure of water at 25°C is 23.8 mmHg.
PO2=752−23.8=728 mmHg=
This corrected pressure is then used in PV=nRT to find the moles of dry gas collected.
Partial Pressure Concepts 🎯
Partial Pressure Calculations 🧮
1) A flask contains 0.50 mol Ar and 1.50 mol Ne. The total pressure is 4.00 atm. What is the partial pressure of Ar? (in atm)
2) Hydrogen gas is collected over water at 22°C (PH2O = 19.8 mmHg). The total pressure is 745 mmHg. What is the pressure of the dry hydrogen? (in mmHg, to 3 significant figures)
3) A mixture has χCO2=0.30 and the total pressure is 2.50 atm. What is PCO2? (in atm)
Dalton's Law Concepts 🔍
Exit Quiz — Dalton's Law ✅
T1
P1V1
=
T2P2V2
One set of conditions → Ideal gas law: PV=nRT
Gas mixtures → Dalton's law: Ptotal=∑Pi
Unknown molar mass → M=mRT/(PV)
Gas density → d=PM/(RT)
Convert all units — Kelvin for T, liters for V, atm for P (if using R=0.0821).
Solve and check — does the answer make physical sense?
The Combined Gas Law
When the amount of gas is constant but P, V, and T all change:
T1P1V1=T2P2V2
This reduces to Boyle's or Charles's law when one variable is held constant.
📌 STP Problems
💡 Tip: At STP (0°C = 273.15 K, 1.00 atm), 1 mol of ideal gas = 22.4 L. This gives a quick shortcut for many calculations!
Example 1: Volume at STP
Problem: What volume does 0.750 mol of CO₂ occupy at STP?
Solution:
V=0.750×22.4=16.8 L
Example 2: Moles from Volume at STP
Problem: A sample of gas occupies 5.60 L at STP. How many moles?
Solution:
n=22.45.60=0.250 mol
Example 3: Combined Gas Law
Problem: A gas at 2.00 atm, 10.0 L, and 300 K is changed to 1.00 atm and 600 K. New volume?
Solution:
V2=T
💨 Gas Stoichiometry
When gases appear in chemical reactions, you can use the ideal gas law with stoichiometry:
grams→moles→mole ratio→moles of gas→PV=nRT
Example
Problem: How many liters of O₂ at 25°C and 1.00 atm are produced from the decomposition of 49.0 g of KClO₃?
Solution:
2KClO3→2KCl+3O2
Step 1: Moles of KClO₃ (M=122.55 g/mol):
n=49.0/122.55=0.400 mol
Step 2: Moles of O₂:
0.400×23=0.600 mol O2
Step 3: Volume:
V=PnRT=
Problem Type Identification 🎯
Calculation Workshop 🧮
1) A 5.00 L gas sample at 2.00 atm and 400 K is cooled to 200 K and compressed to 1.00 L. What is the new pressure? (in atm)
2) At STP, how many grams of CO₂ (M=44.01 g/mol) occupy 11.2 L? (in g, to 3 significant figures)
3) A mixture of 0.30 mol He and 0.70 mol Ar has a total pressure of 5.00 atm. What is the partial pressure of Ar? (in atm)
Quick Decision Guide 🔍
Exit Quiz — Problem Solving ✅
When Do Gases Deviate Most?
High pressure → molecules are close together → volume of molecules matters, attractions are significant
Low temperature → molecules move slowly → attractions have more effect
Near the boiling point → gas is close to condensing → strong intermolecular forces
When Is Ideal Behavior Best?
🔑 Key Concept: Gases behave most ideally at high temperature and low pressure — conditions where molecules are far apart and moving fast.
Low pressure → molecules far apart → negligible volume and attractions
High temperature → fast-moving molecules → overcome attractions easily
Noble gases and small nonpolar molecules → weakest intermolecular forces
📌 The van der Waals Equation
To correct for real gas behavior:
(P+V2an2)(V−nb)=nRT
Correction
Term
What It Fixes
Pressure correction
+an2/V2
Accounts for intermolecular attractions reducing observed pressure
Volume correction
−nb
Accounts for the finite volume occupied by gas molecules
a = attraction parameter (larger for polar molecules with strong IMFs)
b = size parameter (larger for bigger molecules)
Example Values
Gas
a (L²·atm/mol²)
b (L/mol)
He
0.034
0.024
N₂
1.39
0.039
CO₂
3.59
0.043
H₂O
5.46
0.031
Notice: H₂O has a large a (strong H-bonds) but small b (small molecule). He has tiny values for both (noble gas, very small).
Ideal vs. Real Gas Quiz 🎯
AP-Style Calculation Practice 🧮
1) 0.500 mol of an ideal gas at 1.00 atm and 273 K occupies what volume? (in L, to 3 significant figures)
2) A real gas has a=3.59 L²·atm/mol² and b=0.043 L/mol. For 1.00 mol in a 0.500 L container at 500 K, calculate the ideal gas pressure first: Pideal=nRT/V. (in atm, to 3 significant figures)
3) What is the corrected van der Waals pressure for the same gas? Use P=nRT/(V−nb)−an2/V. (in atm, to 3 significant figures)