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Foundation7 past questions

Ideal gas law and gas laws

The ideal gas law and related gas laws describe the relationships between pressure, volume, temperature, and number of moles for ideal gases.

Why this shows up in the exam

You need to apply these laws to solve problems involving gas mixtures, changes of state, and calculations of unknown quantities on NEET.

How NEET tests this

Numerical · 5 QsStatement analysis · 2 QsApplication · 2 Qs

Learn the idea

The ideal gas law links pressure, volume, temperature and amount of gas; using PV = NkT lets you count individual molecules, while PV = nRT works with moles.

🧠 Memory hook: PV = NkT – think “P V equals Number times k times Temperature”, like a tiny R multiplied by each particle.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • PV = nRT (R = 8.314 J mol⁻¹ K⁻¹)
  • PV = NkT (k = 1.38×10⁻²³ J K⁻¹)
  • n = N/Na (Na = 6.022×10²³ mol⁻¹)
  • Average kinetic energy per molecule = (3/2)kT
  • At STP, T = 273 K, P = 1 atm = 101325 Pa, V = 22.4 L per mole
  • 1 L = 0.001 m³

How to approach it

  1. 1Write down what is given and what is asked
  2. 2Convert all quantities to SI units (Pa, m³, K)
  3. 3Choose the form of the gas law that matches the unknown (use PV = NkT when N is required)
  4. 4Solve algebraically, then if needed use Na to switch between N and n

Worked example — watch it click

Temperature of an ideal gas is T K and average kinetic energy is E = 2.07 × 10⁻²³ T J mol⁻¹. Number of molecules in 1 L gas at STP will be :

  • ✅2.68 x 10²²
  • B)2.68 x 10²⁵
  • C)2.68 x 10²⁸
  • D)1.68 x 10²²

The concept behind this problem

The example asks for the number of molecules, so the PV = NkT form is required; the kinetic‑energy formula tells us the temperature‑dependence of kT.

Step by step

  1. 1Average KE per molecule = (3/2)kT.
  2. 2Given E = 2.07×10⁻²³ T J/mol should be per molecule.
  3. 3At STP: T = 273K, P = 1 atm, V = 1L.
  4. 4Using PV = NkT: N = PV/(kT) = (101325 Pa × 0.001 m³)/(1.38×10⁻²³ × 273) ≈ 2.68×10²² molecules.

Watch out

Treating the given kinetic‑energy expression as per‑mole and using R instead of k, which would give the wrong 10²5 magnitude.

Common slip-ups that cost marks

  • •Confusing the gas constant R with Boltzmann constant k
  • •Using Celsius instead of Kelvin for temperature
  • •Ignoring unit conversion for pressure or volume

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 7NEET 2013

In the given (V-T) diagram, what is the relation between pressure P₁ and P₂?

Diagram for this question

Push further

More challenging

15 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 15

At a certain temperature and pressure, 10 liters of an ideal gas has a mass of 20 g. If the molecular weight of the gas is 40 g/mol, what is the value of 'n' in the ideal gas equation PV = nRT for this sample?

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