What is the relationship between the Celsius and the Kelvin scales of temperature?

The molecular theory states that the pressure exerted by a gas in closed vessel results from the gas molecules striking against the walls of the vessel. How will the pressure change if :

(a) the temperature is doubled keeping the volume constant

(b) the volume is made half of its original value keeping the temperature constant?

Define absolute zero and absolute scale of temperature.

(a) Define S.T.P.

(b) Why is it necessary to compare gases at S.T.P.?