--- title: "Gas Volumes and the Ideal Gas Equation" description: "Amount of substance stops being a mass calculation once the sample is a gas. The ideal gas equation ties pressure, volume, temperature and moles together, so a volume measured at the bench becomes a n" canonical: https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD source: https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Gas Volumes and the Ideal Gas Equation Amount of substance stops being a mass calculation once the sample is a gas. The ideal gas equation ties pressure, volume, temperature and moles together, so a volume measured at the bench becomes a number of particles. Subject: Science · Area: Chemistry · Ages 16 to 17 Page: https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD ## Ready when they can - Converts pressure to pascals, volume to cubic metres and temperature to kelvin before substituting anything - Calculates the amount in moles of a gas from measured pressure, volume and temperature - Finds the molar mass of an unknown gas from the mass of a measured volume of it - Says which assumptions of the ideal model break down at high pressure or low temperature ## Lesson: Weighing gases with one equation One equation ties the four measurable properties of a gas together: pV equals nRT. Pressure times volume equals the amount in moles times the gas constant times absolute temperature. Use R as 8.314 joules per mol per kelvin, pressure in pascals, volume in cubic metres, and temperature in kelvin. The amount n comes out in moles. Bench readings need converting before they go in. Multiply kilopascals by 1000 to reach pascals, divide cubic decimetres by 1000 to reach cubic metres, and add 273 to Celsius to reach kelvin. Most slips come from skipping these conversions, not from the algebra. **Example.** Try a sample at 25 C, 150 kilopascals and 2.0 cubic decimetres. Converted, that is 298 kelvin, 150000 pascals and 0.002 cubic metres. Rearrange to n equals pV over RT and the moles drop out. Weigh the flask full and empty for the mass, then divide mass by n to get the molar mass. Real gases stray when particles crowd or slow down. At high pressure each particle occupies a noticeable chunk, so the free space is less than V. Near condensing, attractions tug particles together and the pressure drops below ideal. Point particles with no attractions is the model; squeeze or chill, and it breaks. **Recap.** Convert to pascals, cubic metres and kelvin, then pV equals nRT turns bench readings into moles. Divide a measured mass by that mole count to get the molar mass. Real gases only match this when they are not too cold or too squeezed. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Reacting Masses, Limiting Reactants and Yield](https://lightmysky.com/learn/science/reacting-masses-limiting-reactants-and-yield-mt_C2K6kp5Chv) - [Pressure in Liquids and the Atmosphere](https://lightmysky.com/learn/science/pressure-in-liquids-and-the-atmosphere-mt_eLGNkEJAZl) - [Concentration of Solutions in Moles per Cubic Decimetre](https://lightmysky.com/learn/science/concentration-of-solutions-in-moles-per-cubic-decimetre-mt_vRK78ynnXb) ## Opens up - [Kp, Partial Pressures and Gas Equilibria](https://lightmysky.com/learn/science/kp-partial-pressures-and-gas-equilibria-mt_1R8BpA2hfK) - [Enthalpy Change, Standard Conditions and Calorimetry](https://lightmysky.com/learn/science/enthalpy-change-standard-conditions-and-calorimetry-mt_GPgwgyqyvN) - [Real Gases, Fugacity and Activity](https://lightmysky.com/learn/science/real-gases-fugacity-and-activity-mt_HzRqV-KXfN)