If temperature, volume and pressure of a fixed amount of gas vary from T1 , V1 and p1 to T2 , V2 and p2 then we can write p1 V1/ T1= nR, p2 V2/ T2= n R .

Or p1 V1/ T1= p2 V2/ T2

In this equation out of six, values of five variables are known, the value of unknown variable can be calculated. This equation is known as Combined gas law or Combined gas equation