V1 T2 = V2 T1 is an expression of

Charles' Law

Explanation:

Charles' law gives the relationship between the the volume of a fixed mass of a gas and the absolute temperature.

It states that, Provided the pressure of a fixed mass of a gas remains constant, the volume of a fixed mass of a gas is directly proportional to the absolute temperature.

V/T = constant

When one of the conditions either temperature of volume changes under the same pressure, then the relationship.

V1T2 = V2T1 is true