Why must temperature be in Kelvin in IB Chemistry?

Kelvin is tied to absolute zero, where particle motion (in the model) reaches a minimum. That means Kelvin is proportional to average kinetic energy, which is the physical reason temperature affects pressure and volume. Celsius does not start at “no motion,” so ratios in Celsius break the physics. In calculations, using °C can create impossible results like negative temperatures in a gas equation. In explanations, Kelvin helps you justify why “doubling T doubles average kinetic energy” in the model. When in doubt, convert immediately and write K clearly beside the value.

Which R value should I use for PV=nRT?

In IB Chemistry, you will most commonly use R = 8.314 J mol⁻¹ K⁻¹ with P in Pa and V in m³. Some textbooks also use alternatives (like kPa·dm³ units) to reduce conversions, but the safest exam habit is SI consistency. The real goal is not memorising multiple constants, but keeping units internally consistent. If you do use an alternative R, you must match it to the pressure and volume units exactly. Many lost marks come from mixing “kPa” in the substitution with an SI R value. RevisionDojo's study tools help because worked solutions show unit logic step-by-step, not just the final number.

How do I get faster at PV=nRT questions under time pressure?

Start by memorising the unit checklist, because speed without unit control is just faster wrong answers. Then practise rearranging PV = nRT until you can do it without writing extra algebra. After that, drill short sets of questions that vary one thing at a time (pressure conversions, volume conversions, then combined). Use the IB Chemistry resources page to combine Study Notes, Flashcards, and the Questionbank so you are not guessing what to practise. When you miss a question, ask why: was it the physics idea, the algebra, or the units? Finally, use RevisionDojo’s AI Chat to explain your mistake back to you and generate one fresh, similar question so the fix sticks.

IB Chemistry: The Ideal Gas Equation Explained

IB Chemistry

RevisionDojo Team

•11/21/2025•7 min read

If you have ever stared at PV = nRT and felt like it was less a formula and more a mood, you are not alone. In IB Chemistry, the ideal gas equation is one of those ideas that keeps reappearing in different outfits: stoichiometry today, thermodynamics tomorrow, a data-based question right when you are tired. The good news is that it is always the same relationship underneath. Once you learn to treat it like a reliable tool (and not a trick), gas questions become some of the most predictable marks in the paper.

The ideal gas equation in IB Chemistry (what it really says)

The ideal gas equation is:

PV = nRT

It links four measurable variables to the amount of gas:

P = pressure (Pa is safest)
V = volume (m³ in SI)
n = amount of substance (mol)
R = gas constant (8.314 J mol⁻¹ K⁻¹)
T = temperature (K)

In IB Chemistry, you are also expected to know the model behind it: an ideal gas has particles with negligible volume, no intermolecular attractions, and perfectly elastic collisions. If you want the syllabus-aligned version, start at the S1.5 Ideal gases hub and the S1.5.1 ideal gas model notes.

A quick exam checklist before you calculate

Before you touch your calculator, run this 15-second checklist (it saves more marks than extra studying):

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IB Chemistry: The Ideal Gas Equation Explained

IB Chemistry

RevisionDojo Team

•11/21/2025•7 min read

If you have ever stared at PV = nRT and felt like it was less a formula and more a mood, you are not alone. In IB Chemistry, the ideal gas equation is one of those ideas that keeps reappearing in different outfits: stoichiometry today, thermodynamics tomorrow, a data-based question right when you are tired. The good news is that it is always the same relationship underneath. Once you learn to treat it like a reliable tool (and not a trick), gas questions become some of the most predictable marks in the paper.

The ideal gas equation in IB Chemistry (what it really says)

The ideal gas equation is:

PV = nRT

It links four measurable variables to the amount of gas:

P = pressure (Pa is safest)
V = volume (m³ in SI)
n = amount of substance (mol)
R = gas constant (8.314 J mol⁻¹ K⁻¹)
T = temperature (K)

In IB Chemistry, you are also expected to know the model behind it: an ideal gas has particles with negligible volume, no intermolecular attractions, and perfectly elastic collisions. If you want the syllabus-aligned version, start at the S1.5 Ideal gases hub and the S1.5.1 ideal gas model notes.

A quick exam checklist before you calculate

Before you touch your calculator, run this 15-second checklist (it saves more marks than extra studying):

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Is T in Kelvin? (K = °C + 273)
Is P in Pa? (kPa × 1000)
Is V in m³? (dm³ × 0.001)
Have you chosen one consistent R value and unit set?
Does the final answer scale make sense (tiny moles vs big volume)?

When to use PV=nRT in IB Chemistry

PV = nRT is the bridge between “gas laws” and “moles.” Use it when the question involves any of the following:

Finding moles of a gas from pressure, volume, and temperature
Linking gas volume to stoichiometry (reacting moles)
Converting between mass and volume (via n)
Mixed gas-law changes where more than one variable changes

A useful companion idea is that at constant temperature and pressure, V ∝ n. If that relationship feels shaky, revisit What Is Avogadro's Law? IB Chemistry Explained.

Rearrangements you should know cold

Most mistakes are not chemistry mistakes. They are rearrangement and units mistakes. In IB Chemistry, be able to rearrange quickly:

n = PV / RT
V = nRT / P
P = nRT / V
T = PV / nR

Practise them until they feel automatic, then prove it with timed questions from the S1.5 Ideal gases Questionbank.

Worked example (IB-style, with the unit trap highlighted)

Question: Calculate the number of moles of gas in a 2.50 dm³ container at 100 kPa and 298 K.

Step 1: Convert units

V = 2.50 dm³ = 2.50 × 0.001 = 0.00250 m³
P = 100 kPa = 100 × 1000 = 100,000 Pa

Step 2: Use PV = nRT
[ n = \frac{PV}{RT} = \frac{(100000)(0.00250)}{(8.314)(298)} ]

n ≈ 0.101 mol

That single conversion (dm³ to m³) is often the difference between a clean mark and an answer off by a factor of 1000. If you want structured practice with step-by-step feedback, the S1.5 Ideal gases lessons and S1.5 videos are built for exactly these patterns.

What each variable means (so you can explain, not just calculate)

IB markschemes love “explain” questions. The equation helps you narrate what happens:

Increase T (at constant V) and particles hit the walls faster and harder, so P increases.
Decrease V (at constant T) and collisions become more frequent per unit area, so P increases.
Increase n (at constant T and V) and there are more particles colliding, so P increases.

For a clean explanation of collision reasoning, pair this with Why does pressure increase when gas particles collide more frequently and the broader intuition in IB Chemistry: Why Gases Expand to Fill Any Container.

Limitations (when real gases stop playing nice)

The ideal gas equation works well at moderate temperatures and pressures. Real gases deviate most when:

Pressure is very high (particles are forced close together)
Temperature is very low (attractions matter more)
Molecules are large or polar (stronger intermolecular forces)

In most IB Chemistry exam contexts, PV = nRT is still the expected tool unless the question explicitly signals real-gas behaviour.

Closing: make PV=nRT your easiest marks

The ideal gas equation is not just a formula to memorise. In IB Chemistry, it is a quiet organiser: it turns a messy word problem into a few consistent units and a single relationship. If you build the habit of checking units first, rearranging cleanly, and explaining changes using particle collisions, PV = nRT becomes one of the most reliable scoring topics.

To lock it in, use RevisionDojo as your full system: syllabus-aligned Study Notes, rapid Flashcards, exam-style Mock Exams, Predicted Papers, and the Questionbank for repetition, plus Grading tools, the Coursework Library, and Tutors when you want targeted feedback. Start with the S1.5 Ideal gases notes, then test yourself until IB Chemistry gas questions feel routine.

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