A gas occupies 3.0 L at 27°C. What volume does it occupy at 54°C at the same pressure and amount? (Assume ideal behavior.)
A6.0 L — temperature doubled (54/27 = 2), so volume doubles
B1.5 L — temperature increased, so pressure increases and volume decreases
C3.27 L — using Kelvin: V₂ = 3.0 × (327 K / 300 K)
D4.0 L — temperature increased by 27°C, which is one-ninth of 27°C, so volume increases by one-ninth
Charles's law requires absolute temperature: V₁/T₁ = V₂/T₂ in Kelvin. Convert: 27°C = 300 K, 54°C = 327 K. Then V₂ = 3.0 L × (327/300) = 3.27 L. The Celsius-trap answer (option A) multiplies by 54/27 = 2, doubling the volume — this is wrong because Celsius zero is not absolute zero. Temperature only doubled in Celsius (from 27 to 54); in Kelvin it increased by only about 9% (from 300 to 327). Using Celsius in gas law calculations produces wildly incorrect results.
Question 2 Multiple Choice
Under which conditions do real gases deviate most significantly from ideal gas behavior?
AHigh temperature and low pressure
BLow temperature and high pressure
CAny temperature at very low amounts of gas
DConditions where the gas has a low molar mass
The ideal gas law assumes (1) gas molecules have negligible volume and (2) there are no intermolecular forces. Both assumptions break down under extreme conditions. At high pressure, molecules are squeezed close together — their own volume becomes a significant fraction of the total volume. At low temperature, molecules move slowly enough that intermolecular attractive forces can pull them together, reducing pressure below the ideal prediction. Real gases behave most ideally when far apart and moving fast: low pressure and high temperature.
Question 3 True / False
According to Charles's law, a gas sample at exactly 0°C would have zero volume.
TTrue
FFalse
Answer: False
This is the classic Celsius error. Charles's law states V ∝ T only when T is in Kelvin. Zero volume corresponds to absolute zero, which is 0 K = −273.15°C. A gas at 0°C is actually at 273.15 K, and it still occupies a substantial volume. Using Celsius in gas law calculations as if 0°C = zero volume is precisely the error the Kelvin requirement prevents. Lord Kelvin defined the absolute temperature scale specifically so that gas laws work correctly: 0 K is where an ideal gas would have zero volume.
Question 4 True / False
At the same temperature and pressure, equal volumes of different ideal gases contain the same number of molecules, regardless of the identity of the gas.
TTrue
FFalse
Answer: True
This is Avogadro's law: V ∝ n at constant T and P, which means equal volumes at the same T and P contain equal n (moles), and therefore equal numbers of molecules. It doesn't matter whether the gas is H₂, O₂, CO₂, or Ar — the identity of the gas is irrelevant to this relationship for ideal gases. This principle underpins STP stoichiometry: 22.4 L/mol applies to any ideal gas at 0°C and 1 atm, which is a direct consequence of Avogadro's law combined with the ideal gas equation.
Question 5 Short Answer
Why must temperature always be converted to Kelvin when using the gas laws, and what physically incorrect prediction would you get if you used Celsius instead?
Think about your answer, then reveal below.
Model answer: The gas laws require an absolute temperature scale because they describe proportional relationships between gas properties and the average kinetic energy of molecules. Kelvin is absolute: 0 K means zero molecular motion (theoretically). Celsius is an arbitrary offset scale where 0°C is the freezing point of water — not zero molecular motion. Using Celsius, Charles's law (V ∝ T) would predict that a gas at 0°C has zero volume — physically impossible and obviously wrong. Converting 0°C to 273.15 K gives the correct nonzero volume. More generally, Celsius arithmetic gives the wrong ratio: doubling from 10°C to 20°C is only a 3.5% increase in Kelvin (283 K to 293 K), not a doubling of volume.
The Kelvin scale makes the mathematics work because it starts at the physically meaningful zero (no thermal energy). Any other scale's zero is arbitrary and breaks the proportionality. Temperature in gas laws is a proxy for kinetic energy, and kinetic energy only goes to zero at absolute zero — not at 0°C.