Gas relationships and disciplined unit use

Gas relationships and disciplined unit use

Gas-law reasoning works when you identify which variables are held constant. At constant pressure and amount, volume is proportional to kelvin temperature. At constant temperature and amount, volume decreases when pressure increases because the same number of particles collide with a smaller wall area more often. At constant volume, raising temperature increases pressure because collisions become more energetic. Use a ratio only when the stated conditions justify it. In every calculation, write temperature in kelvin and keep volume units consistent; a correct-looking number with mismatched units is not a chemistry answer.

Work it through

A gas occupies 2.0 dm³ at 300 K under constant pressure. At 450 K, V₂/V₁ = T₂/T₁, so V₂ = 2.0 × 450/300 = 3.0 dm³. The result makes sense because the gas is hotter at the same pressure and expands. If the temperature had been entered as 27 and 177 without conversion, the ratio would be meaningless.

Mastery target

Choose the valid gas relationship from the stated controls, calculate with kelvin temperatures and explain the answer using collision theory.