Question

Temperature remaining constant the product of volume and pressure pf a given mass of dry gas is a constant. Give reason

Answer 1

The relation between pressure and volume at constant temperature is best described As Boyle's Law, which says that at constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume.
Or in other words we can say that the product of pressure and volume is constant, pV = constant.

Suppose if a fixed amount gas occupies a volume V1 and pressure p1 at a constant temperature, and it undergoes expansion so that the volume will become V2 and pressure p2, then the product of P and V will always be a constant value.

p1V1 = p2​V2 = constant.

Answer 2

If pressure remains constant :

Then 

According to Boyle's law 

V α 1/P

and 

V = K x 1/P 

OR VP = k 

hence product of pressure and volume is constant