Ideal gas — Science definition

A hypothetical gas that perfectly obeys the KMT assumptions; real gases behave most ideally at low pressure and high temperature.

Where it is taught

In use

The ideal gas law closes that gap by adding the amount of gas, in moles, right into the equation.

The Ideal Gas Law

Questions using ideal gas

Problem 1

In the ideal gas law PV = nRT, what does n represent?
  1. the mass in grams
  2. the number of collisions
  3. the number of moles of gas
  4. the number of neutrons
Answer and explanation

the number of moles of gas

n is the amount of gas measured in moles.

Problem 2

What volume does 3.0 mol of an ideal gas occupy at STP?
Answer and explanation

67.2 L

Volume = moles × 22.4 L/mol = 3.0 × 22.4 = 67.2 L.

Problem 3

Density connects amount and mass without replacing either quantityDensity connects amount and mass without replacing either quantityn = m/MₘPV = (m/Mₘ)RTρ = m/V = PMₘ/(RT)Mₘ = ρRT/PA density comparison must keep P, T and units compatible.
Original algebraic derivation; it does not assume liquid density follows the same temperature law.
Interpret the supplied visual. If temperature increases at fixed pressure for the same ideal gas, what happens to its density?
Answer and explanation

It decreases inversely with absolute temperature because the amount per volume decreases under the model.

Substituting n=m/Mₘ into the ideal gas law gives ρ=PMₘ/(RT). Gas density therefore depends on pressure and temperature as well as chemical identity. Rearranging to Mₘ=ρRT/P can estimate a molar mass when the sample is sufficiently ideal and its composition is appropriate. A mixture’s result can represent an average rather than one pure species. State the gas conditions and uncertainty before interpreting a density as unique substance identification.

It decreases inversely with absolute temperature because the amount per volume decreases under the model.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 4

Which equation is the ideal gas law?
  1. PV = RT
  2. PV = nRT
  3. P = mV
  4. V = nRT
Answer and explanation

PV = nRT

The ideal gas law is PV = nRT.

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