Why is ideal gas law in kelvin?
For all gas law problems it is necessary to work in the Kelvin scale because temperature is in the denominator in the combined gas laws (P/T, V/T and PV/T) and can be derived in the ideal gas law to the denominator (PV/RT) . If we measured temperature in celsius we could have a value of zero degrees celsius and this would solve as no solution, as you cannot have zero in the denominator.
However, if we reached zero in the Kelvin scale this would be absolute zero and all matter would stop and therefore there would be no gas laws to worry about.
This is of course an over simplification of the situation, but it reminds us mathematically why we work in Kelvin instead of Celsius, specially for the gas laws.
I hope this was helpful.