15.2.1—A gas obeying pV ∝ T, where T is the thermodynamic temperature, is known

At fixed amount and volume, an ideal gas obeys p∝T, so p/T is constant when T is measured in kelvin.

Use absolute temperature and keep volume and amount fixed. A pressure change can be compared through p₁/T₁=p₂/T₂.

Heating a sealed rigid container from 300 K to 450 K raises ideal-gas pressure by a factor of 1.5.

The proportionality fails if volume or amount changes, and Celsius temperature cannot be used in the ratio.