Calculation Traps

1. Using Celsius instead of Kelvin. Every gas law calculation requires absolute temperature. $T(K) = T(°C) + 273$. A question stating "27°C" always means 300 K. This is the single most common arithmetic error in NEET gas law problems.

2. Inverting Graham's Law. The formula is $r_1/r_2 = \sqrt{M_2/M_1}$ — the heavier molar mass is in the numerator of the fraction inside the root. Students frequently write $\sqrt{M_1/M_2}$ instead. A quick sanity check: the lighter gas should always have the faster rate.

3. Wrong value of R. Using $R = 8.314$ with pressure in atm (instead of 0.0821) gives an answer off by a factor of $\sim 101$. Match R units to the pressure units given.

4. Confusing $v_{rms}$, $v_{avg}$, $v_{mp}$ formulas. The coefficients under the root are 3, 8/π (≈2.55), and 2 respectively. The one with coefficient 3 (largest) is $v_{rms}$. Mnemonic: RMS = Rightmost Maximum Speed (largest value).

Conceptual Traps

5. Assuming $H_{2}$ and He have $Z < 1$ at moderate pressures. They are exceptions — $Z > 1$ at ALL pressures because their $a$ is negligibly small, so volume correction dominates even at low pressures.

6. Confusing real gas corrections. $a$ corrects for attraction (added to pressure: $P + a/V^2$ because attraction reduces the observed pressure), $b$ corrects for molecular volume (subtracted from volume: $V - b$).

7. Mixing up effusion time and rate. Rate $\propto 1/\sqrt{M}$, so rate is FASTER for lighter gas. But time for fixed amount to effuse $\propto \sqrt{M}$, so TIME is LONGER for heavier gas: $t_1/t_2 = \sqrt{M_1/M_2}$.

8. Stating viscosity increases with temperature for liquids. True for gases (more molecular collisions), but FALSE for liquids (intermolecular forces weaken with T, reducing resistance to flow). NEET almost always tests the liquid case.