MHT-CET Chemistry · Teaching notes

States of Matter — MHT-CET Chemistry

The gas-phase chapter of MHT-CET Chemistry — a compact, calculation-friendly topic (32 PYQs) built on the gas laws and one master equation, PV = nRT. It teaches in two movements: (1) the gas laws and the ideal gas equation — Boyle's, Charles', Gay-Lussac's and the combined gas law, then PV = nRT with its unit-matched R values; (2) real gases, Dalton's law of partial pressures and the kinetic theory of gases — partial pressure from mole fraction, root-mean-square speed, the KTG postulates and van der Waals deviation. Almost every question is a one- or two-step plug-in; the recurring traps are units (kelvin, matching R) and remembering that partial pressure tracks moles, not mass. Every PYQ tagged.

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Subtopic notes

PYQ weightage by concept

10 concepts · 32 PYQs — where the marks actually sit, so you know what to drill first

Gas Laws and the Ideal Gas Equation19 PYQs · 59%

Concept	PYQs	Share
Boyle's law — pressure and volume	5	16%
Charles' law — volume and temperature	4	13%
Ideal gas equation, PV = nRT	4	13%
Equal masses in equal volumes — lightest gas, highest pressure	3	9%
Gay-Lussac's law — pressure and temperature	2	6%
Combined gas law	1	3%

Real Gases, Dalton's Law and the Kinetic Theory of Gases13 PYQs · 41%

Concept	PYQs	Share
Dalton's law of partial pressures	7	22%
Real gases and the compressibility factor	4	13%
Root-mean-square velocity	1	3%
Postulates of the kinetic theory of gases	1	3%

Formula & revision sheet

9 formulas · 1 reference tables · 19 gotchas across all subtopics — the exam-eve cheat-sheet

Gas Laws and the Ideal Gas Equation

Formulas (6)

Boyle's law — pressure and volume · Boyle's law $P_1 V_1 = P_2 V_2 \qquad (T,\, n \text{ constant})$
Charles' law — volume and temperature · Charles' law $\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2} \qquad (P,\, n \text{ constant})$
Gay-Lussac's law — pressure and temperature · Gay-Lussac's law $\dfrac{P_1}{T_1} = \dfrac{P_2}{T_2} \qquad (V,\, n \text{ constant})$
Combined gas law · Combined gas law $\dfrac{P_1 V_1}{T_1} = \dfrac{P_2 V_2}{T_2}$
Ideal gas equation, PV = nRT · Ideal gas equation $PV = nRT = \dfrac{m}{M}RT$
Equal masses in equal volumes — lightest gas, highest pressure · Pressure vs molar mass (equal mass, V, T) $P \propto \dfrac{1}{M} \qquad \left(n = \dfrac{m}{M}\right)$

Watch out for (11)

No unit conversion inside Boyle's law
→ Boyle's law — pressure and volume
Boyle's law is P vs V — not PV vs P
→ Boyle's law — pressure and volume
Kelvin, always — never Celsius in the ratio
→ Charles' law — volume and temperature
Absolute zero is negative
→ Charles' law — volume and temperature
Do not mix up the three simple laws
→ Gay-Lussac's law — pressure and temperature
Absolute temperature here too
→ Gay-Lussac's law — pressure and temperature
T on the DENOMINATOR, in kelvin
→ Combined gas law
Match R's units to the pressure and volume
→ Ideal gas equation, PV = nRT
Watch a printed exponent typo
→ Ideal gas equation, PV = nRT
Equal MASS, not equal moles
→ Equal masses in equal volumes — lightest gas, highest pressure
Lightest gas = highest pressure
→ Equal masses in equal volumes — lightest gas, highest pressure

Real Gases, Dalton's Law and the Kinetic Theory of Gases

Formulas (3)

Dalton's law of partial pressures · Partial pressure from mole fraction $P_i = x_i\,P_{\text{total}} = \dfrac{n_i}{n_{\text{total}}}\,P_{\text{total}}$
Root-mean-square velocity · Root-mean-square velocity and its ratio $v_{rms} = \sqrt{\dfrac{3RT}{M}}\qquad \dfrac{v_1}{v_2} = \sqrt{\dfrac{T_1/M_1}{T_2/M_2}}$
Real gases and the compressibility factor · Compressibility factor and van der Waals equation $Z = \dfrac{PV}{nRT}\qquad \left(P + \dfrac{an^2}{V^2}\right)(V - nb) = nRT$

Reference tables (1)

Postulates of the kinetic theory of gases5 rows

Postulate	Statement
Negligible molecular volume	The actual volume of the gas molecules is negligibly small compared with the total volume of the container; the gas is mostly empty space. This assumption fails at high pressure, when molecules are squeezed close together and their own volume is no longer negligible.
No intermolecular forces	There are no forces of attraction or repulsion between the molecules of an ideal gas; they move completely independently. This assumption fails at low temperature / high pressure, when attractions pull molecules together — the reason gases can be liquefied.
Elastic collisions	Collisions between molecules, and with the walls, are perfectly elastic — the total kinetic energy is conserved during every collision.
Kinetic energy proportional to temperature	The average kinetic energy of the molecules is directly proportional to the absolute temperature; it depends only on T, not on the gas's identity.
Continuous random motion	Molecules are in constant, rapid, random straight-line motion in all directions, colliding with one another and the container walls.

The two bold postulates (zero volume, zero force) are what an ideal gas assumes and a real gas violates.

Watch out for (8)

Partial pressure follows moles, not mass
→ Dalton's law of partial pressures
Use the total moles in the denominator
→ Dalton's law of partial pressures
Take the square root at the end
→ Root-mean-square velocity
Use absolute temperature in kelvin
→ Root-mean-square velocity
Kinetic energy depends on temperature, not on the gas
→ Postulates of the kinetic theory of gases
Ideal gas = zero volume AND zero force
→ Postulates of the kinetic theory of gases
Z = 1 means ideal, in either direction
→ Real gases and the compressibility factor
Multiply, do not add, the ideal molar volume
→ Real gases and the compressibility factor