a fixed amount of ideal gas is held in a rigid container that expands negligibly when heated. at 20°c the gas pressure is p. if we add enough heat to increase the temperature from 20°c to 40°c, the pressure will be:

In the ideal gas law, we have to use temperature in Kelvin. Otherwise we would very quickly have problems.

For example, given that P/T is a constant, suppose T was in Celcius and we wanted to know the pressure for a balloon of hydrogen gas going from 1 atm at 20 C to -10 C. Then we'd have

P1T1=P2T2⟹P2=P1T1T2=120(−10)=−1/2

a negative pressure! Which is clearly nonsense.

Hence you need to convert the temperatures 20 and 40 C into an absolute temperature scale, Kelvin.

E.g. 20 C = 273 + 20 K = 293 K