NEET Chemistry — States of Matter Complete Chapter Guide

Chapter Overview & Weightage

States of Matter focuses on gas laws, kinetic theory, and real gas behavior. NEET tests this chapter with formula-based problems — if you know the equations, the marks are yours.

States of Matter carries 3-4% weightage in NEET. Ideal gas law, Graham’s law, and van der Waals equation are the most commonly tested areas.

Year	NEET Q Count	Key Topics Tested
2025	2	Ideal gas, Graham’s law
2024	1	van der Waals, critical constants
2023	2	KMT, RMS speed
2022	2	Gas laws, compressibility factor
2021	1	Dalton’s law, ideal gas

graph TD
    A[States of Matter] --> B[Gas Laws]
    A --> C[Kinetic Molecular Theory]
    A --> D[Real Gases]
    B --> E[Boyle's Law]
    B --> F[Charles's Law]
    B --> G[Ideal Gas: PV = nRT]
    B --> H[Dalton's Partial Pressures]
    C --> I[RMS, Average, Most Probable Speed]
    C --> J[Graham's Law of Diffusion]
    D --> K[van der Waals Equation]
    D --> L[Compressibility Factor Z]

Key Concepts You Must Know

Tier 1 (Always asked)

Ideal gas equation: $PV = nRT$
Graham’s law: rate of diffusion $\propto 1/\sqrt{M}$
Relationship between molecular speeds: $v_{mp} : v_{avg} : v_{rms} = 1 : 1.128 : 1.224$
Dalton’s law of partial pressures

Tier 2 (Frequently asked)

van der Waals equation: $(P + a/V^2)(V - b) = RT$
Compressibility factor: $Z = PV/(nRT)$ . $Z = 1$ for ideal, $Z < 1$ for attractive domination, $Z > 1$ for repulsive domination
Critical constants in terms of $a$ and $b$

Tier 3 (Occasional)

Boyle temperature
Liquefaction conditions
Mean free path

Important Formulas

PV = nRT

$R = 8.314$ J/mol/K $= 0.0821$ L atm/mol/K

Boyle’s law: $P_1V_1 = P_2V_2$ (constant $T$ , $n$ )

Charles’s law: $V_1/T_1 = V_2/T_2$ (constant $P$ , $n$ )

Dalton’s law: $P_{total} = p_1 + p_2 + p_3 + \ldots$

RMS speed: $v_{rms} = \sqrt{\dfrac{3RT}{M}}$

Average speed: $v_{avg} = \sqrt{\dfrac{8RT}{\pi M}}$

Most probable speed: $v_{mp} = \sqrt{\dfrac{2RT}{M}}$

Graham’s law: $\dfrac{r_1}{r_2} = \sqrt{\dfrac{M_2}{M_1}}$

(P + \frac{an^2}{V^2})(V - nb) = nRT

$a$ corrects for intermolecular attraction, $b$ for molecular volume.

Critical constants: $T_c = \dfrac{8a}{27Rb}$ , $P_c = \dfrac{a}{27b^2}$ , $V_c = 3b$

Compressibility factor: $Z = \dfrac{PV_m}{RT}$

For speed ratio problems, remember: $v_{mp} : v_{avg} : v_{rms} = \sqrt{2} : \sqrt{8/\pi} : \sqrt{3} \approx 1 : 1.128 : 1.224$ . The most probable speed is the smallest, RMS is the largest. This ordering shows up in NEET conceptual questions.

Solved Previous Year Questions

PYQ 1 — NEET 2023

Problem: The RMS speed of O $_2$ at $T$ K is the same as that of H $_2$ at 300 K. Find $T$ .

Solution:

v_{rms} = \sqrt{\frac{3RT}{M}}

Setting RMS speeds equal:

\sqrt{\frac{3RT}{32}} = \sqrt{\frac{3R \times 300}{2}}

\frac{T}{32} = \frac{300}{2} \implies T = \frac{300 \times 32}{2} = \mathbf{4800 \text{ K}}

PYQ 2 — NEET 2022

Problem: The compressibility factor $Z$ for an ideal gas is:

Solution:

For an ideal gas, $PV = nRT$ , so:

Z = \frac{PV_m}{RT} = \mathbf{1}

$Z > 1$ means the gas is harder to compress (repulsive forces dominate). $Z < 1$ means attractive forces dominate.

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	45%	Direct formula, gas law substitution
Medium	40%	Graham’s law, speed comparisons
Hard	15%	van der Waals, critical constants

Expert Strategy

Week 1: Master the ideal gas equation and all its sub-laws. Practise unit conversions between atm/Pa/mmHg and L/mL/m $^3$ .

Week 2: Molecular speeds and Graham’s law. Most problems boil down to ratio-and-proportion with molar masses.

Week 3: van der Waals equation and compressibility factor — these are less frequent but carry higher marks.

Common Traps

Trap 1 — Unit of R. Using $R = 8.314$ when pressure is in atm and volume in litres gives wrong answers. Use $R = 0.0821$ L atm mol $^{-1}$ K $^{-1}$ in that case.

Trap 2 — Graham’s law uses molar mass, not molecular mass. The formula is $r_1/r_2 = \sqrt{M_2/M_1}$ where $M$ is molar mass. Using atomic mass instead of molar mass gives wrong ratios for polyatomic molecules.

Trap 3 — Temperature must be in Kelvin. All gas law calculations require absolute temperature. $0°C = 273$ K, not 0 K. Adding 273 is a step students skip under pressure.