P/T=K, a masa y volumen constante

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