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3.3Properties of Substances and Mixtures

Gas Laws — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 8 of 20 1 quiz

📚 AP CHEMISTRY Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

3.3 Gas Laws — AP Notes — AP CHEMISTRY notes: Relate the macroscopic properties of a gas using the ideal gas law and related laws.PV = nRT, with P in atm, V in L, n in mol, T in K and R = 0.08206 L atm mol−1 K−1.What the symbols mean: p = pressure (Pa) · V = volume (m3) · n = amo…

  • Pressure conversions
  • Molar volume at STP (0 °C, 1 atm)
  • Combined gas law
  • Gas density
  • Molar mass
  • Dalton's law

Section 1 of 10Ideal gas law

Ideal gas law
PV
PV nRT, with P in atm, V in L, n in mol, T in K and R = 0.08206 L atm mol−1 K−1.
What the symbols mean: p = pressure (Pa) · V = volume (m3) · n = amount of gas (mol) · R = 8.31 J mol−1 K−1 · T = temperature (K = °C + 273)
It combines the Boyle, Charles, Gay-Lussac and Avogadro relationships and describes a gas by P, V, n and T independent of its identity.
Temperature must be absolute because P and V are directly proportional to kelvin temperature, not Celsius.
Pressure conversions
  • 1 atm = 760 mmHg = 760 torr = 101.325 kPa.
Molar volume at STP (0 °C, 1 atm)
22.4 L mol−1.

Section 2 of 10Particle explanations

Particle explanations
More gas particles at constant V and T give more wall collisions, so P increases.
Decreasing the volume at constant T increases the collision frequency, so P increases.
Increasing T at constant V makes particles move faster, so collisions are more frequent and more forceful and P increases.

Section 3 of 10Combined, density and molar mass

Combined gas law
  • P1V1/T1 = P2V2/T2.
Gas density
  • D = PM/RT, so density is proportional to molar mass at fixed P and T.
Molar mass
  • M = mRT/PV, found by vaporising a volatile liquid in a flask of known volume at known T and P.

Section 4 of 10Mixtures of gases

Mixtures of gases
Dalton's law
  • Ptotal = PA + PB + PC + ...
Partial pressure
  • PA = XA × Ptotal.
Each gas in a mixture behaves independently, so total pressure depends only on total moles.
Gas collected over water
  • Pgas = Patm − PH2O at that temperature.
When two flasks are connected, each gas expands into the total volume and its new partial pressure = original pressure × (its original volume ÷ total volume).

Section 5 of 10Gas stoichiometry

1
Write the balanced equation.
2Convert gas volume to moles with PV
Convert gas volume to moles with PV nRT, or 22.4 L mol−1 at STP.
3
Use the mole ratio.
4
Convert to the required quantity.

Section 6 of 10Absolute zero

1
A plot of volume against temperature in Celsius is a straight line extrapolating to zero volume at −273.15 °C.
2This defines absolute zero, 0 K, and K
°C + 273.15.

Section 7 of 10Kinetic molecular theory and real gases

Assumptions
  • Assumptions particles have negligible volume, move randomly in straight lines, exert no forces except in collisions, collide elastically,
    • and have average kinetic energy proportional to temperature.
Real gases deviate at high pressure (measured volume greater than ideal,
  • because of particle volume) and low temperature (measured pressure less than ideal,
  • because of attractions).
For an ideal gas PV/nRT
1 at every pressure; real gases give below 1 when attractions dominate and above 1 when particle volume dominates.

Section 8 of 10More from the specification

More from the specification
Absolute temperature scale and gas volume
  • Absolute temperature scale and gas volume calibrating gas thermometers against the kelvin scale provides an experimental means to verify Charles's law.
Molecule
  • Two or more atoms held together by covalent bonds.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
Theory
  • A tested hypothesis that is widely accepted.
Oxygen
  • A gas that relights a glowing splint.
Boltzmann distribution
  • The distribution of molecular energies in a sample at a fixed temperature.
Intermolecular force
  • Weak force between molecules.

Section 9 of 10Key terms

TermMeaning
Ideal gas lawPV = nRT.
Dalton's lawTotal pressure is the sum of partial pressures.
Partial pressurePressure a gas would exert alone.
Absolute zero0 K, where an ideal gas would have zero volume.
STP0 °C and 1 atm.
Gas densityD = PM/RT.

Section 10 of 10Exam technique

1
Using Celsius in PV = nRT — use kelvin
2
Using Patm for a gas collected over water without subtracting vapour pressure
3
Using an R value whose units do not match the pressure and volume
4
Saying decreased volume increases the force of each collision — it increases collision frequency

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