Gases — Cheat Sheet

GAS LAWS (constant n)

Boyle's

P₁V₁ = P₂V₂ (const T)

Charles's

V₁/T₁ = V₂/T₂ (const P)

Gay-Lussac

P₁/T₁ = P₂/T₂ (const V)

Combined

P₁V₁/T₁ = P₂V₂/T₂

ALWAYS convert T to Kelvin:
K = °C + 273.15

STANDARD CONDITIONS

STP

0°C (273 K), 101.325 kPa (1 atm)

SATP

25°C (298 K), 100 kPa

RELATIONSHIPS

Boyle's: P & V inverse (T fixed)
Charles's: V & T direct (P fixed)
Gay-Lussac: P & T direct (V fixed)
Combined: all three at once (n fixed)

IDEAL GAS LAW

PV = nRT

pressure (kPa or atm)

volume (L)

moles (mol)

temperature (K)

R (kPa)

8.314 L·kPa/(mol·K)

R (atm)

0.08206 L·atm/(mol·K)

MOLAR VOLUME

At STP: 1 mol = 22.4 L for any ideal gas

USING PV = nRT

Find V

V = nRT / P

Find n

n = PV / RT

Find P

P = nRT / V

Find T

T = PV / nR

DALTON'S LAW

Total P

Pₜ = P₁ + P₂ + P₃ + …

Mole fraction

χ₁ = n₁/nₜ = P₁/Pₜ

Over water

P☟ₐⸯ = Pₜₒₜ − Pₕ₂ₒ

UNIT CONVERSIONS

Pressure	Value
1 atm	101.325 kPa
1 atm	760 mmHg = 760 torr
1 kPa	1000 Pa

Volume	Value
1 L	1000 mL = 1000 cm³
1 m³	1000 L

Temperature	Formula
°C → K	K = °C + 273.15
K → °C	°C = K − 273.15

KINETIC MOLECULAR THEORY

Particles in constant random motion
Negligible particle volume
No intermolecular forces (ideal)
Elastic collisions
Avg KE ∝ T (Kelvin)

Higher T → faster particles → more/harder collisions → higher P (if fixed V)

COMMON MISTAKES

ALWAYS use Kelvin for T in gas laws
Use combined law when two variables change
R value depends on pressure units (kPa vs atm)
Subtract Pₕ₂ₒ when collecting over water
Real gases deviate at high P, low T

WHICH LAW TO USE?

Situation	Law
Only P and V change	Boyle's
Only V and T change	Charles's
Only P and T change	Gay-Lussac
P, V, T all change	Combined
Moles involved	Ideal Gas Law
Gas mixture	Dalton's Law