Ideal Gas Equation — Chemistry Mnemonic
Target: mnemonic for ideal gas equation PV nRT chemistry
Why is this hard to memorize?
The ideal gas equation PV = nRT combines Boyle's law (P∝1/V), Charles's law (V∝T), and Avogadro's law (V∝n) into one master equation. P = pressure, V = volume, n = moles, R = gas constant, T = temperature (Kelvin only!). Use R = 0.0821 L·atm/(mol·K) when P is in atm and V in litres, or R = 8.314 J/(mol·K) for SI units. Applications: finding molar mass (M = mRT/PV), gas density (ρ = PM/RT), and mixture problems (Dalton's law of partial pressures). NEET and JEE test unit-matching and multi-step calculations.
Classic mnemonics you should know
"PV = nRT — "Pavan Never Runs Twice" or just remember: Pressure × Volume = moles × R × Temperature"
This single equation replaces Boyle, Charles, and Avogadro laws. Always use Kelvin for temperature (K = °C + 273). Match R to your units: R = 0.0821 for atm/L, R = 8.314 for Pa/m³.
"n = mass/M, so PV = (m/M)RT → M = mRT/PV. "Find M from gas data""
Substitute n = m/M into PV = nRT: PV = (m/M)RT → M = mRT/(PV). This lets you find molar mass from experimental gas data (pressure, volume, mass, temperature). Very common NEET calculation.
"ρ = m/V = PM/(RT). Gas density depends on P, M, and T — "heavier gas = denser, hotter = less dense""
From PV = nRT and n = m/M: PV = (m/M)RT → m/V = PM/RT → ρ = PM/RT. At constant T and P, density is proportional to molar mass. This explains why CO₂ sinks and H₂ rises.
The complete list
- PV = nRT (ideal gas equation)
- R = 0.0821 L·atm/(mol·K)
- R = 8.314 J/(mol·K)
- M = mRT/(PV) (molar mass from gas data)
- ρ = PM/(RT) (gas density)
- Dalton: P_total = P₁ + P₂ + P₃...
- Graham: rate ∝ 1/√M (diffusion)
- Always use Kelvin for T!
Frequently asked questions
When does the ideal gas equation fail?
Real gases deviate from ideal behaviour at: (1) High pressure — molecules are close together, volume of molecules matters. (2) Low temperature — molecules move slowly, intermolecular attractions matter. Ideal gas assumes: zero molecular volume and zero intermolecular forces. Van der Waals equation corrects for both: (P + a/V²)(V − b) = nRT, where a = attraction correction, b = volume correction.
What is Dalton's law of partial pressures?
In a mixture of non-reacting gases, total pressure = sum of partial pressures of each gas. Partial pressure of gas A: Pₐ = xₐ × P_total (where xₐ = mole fraction). Example: air is 78% N₂ and 21% O₂ at 1 atm. P(N₂) = 0.78 × 1 = 0.78 atm. P(O₂) = 0.21 × 1 = 0.21 atm.
How do I use Graham's law of diffusion?
Rate of diffusion ∝ 1/√M (lighter gas diffuses faster). For two gases: r₁/r₂ = √(M₂/M₁). Example: H₂ (M=2) diffuses √(32/2) = 4 times faster than O₂ (M=32). This explains why hydrogen balloons deflate faster than air balloons, and is used in uranium enrichment (UF₆ diffusion).
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