Mistake 1: Using Celsius instead of Kelvin Wrong: 27°C used directly in PV = nRT. Right: Convert 27°C → 300 K. EVERY gas law uses T in Kelvin.

Mistake 2: Inverting Graham's Law ratio Wrong: r_{1}/r_{2} = √($M_{1}$/$M_{2}$) → heavy gas appears faster. Right: r_{1}/r_{2} = √($M_{2}$/$M_{1}$) → lighter gas is faster (heavier M in numerator). Memory aid: "lighter goes faster" — M of the heavier gas goes on top.

Mistake 3: Confusing v_rms > v_avg > v_mp ordering Common error: writing v_mp > v_avg > v_rms (reversed). Right: v_rms is always the LARGEST (1.224 : 1.128 : 1). Mnemonic: RAM — Rms, Average, Most probable (descending order).

Mistake 4: Z < 1 means gas is non-ideal — not specifying direction NEET often asks: "Z < 1 means...?" Answer: attractive forces dominate → gas more compressible than ideal → actual V < ideal V.

Mistake 5: Applying Boyle's Law when temperature changes Boyle's Law holds ONLY at constant T. If T also changes, must use combined gas law.

Mistake 6: Equal masses ≠ equal moles in partial pressure problems 1 g $H_{2}$ gives 0.5 mol; 1 g $O_{2}$ gives only 0.031 mol. Molar mass matters.

Mistake 7: Forgetting to convert mL to L 500 mL = 0.5 L; 250 mL = 0.25 L. Always convert volume to litres when using R = 0.0821 L·atm/(mol·K).

Mistake 8: Assuming all gases show Z < 1 at moderate pressures $H_{2}$ and He ALWAYS show Z > 1 (their 'a' is too small to cause Z < 1 dip).