How can the Change in Mass be used to measure the Rate of Reaction?

-Using a Mass Balance, and a Reaction that makes a Gas Product, you can measure the Rate of Reaction -As the Gas is Released, the Mass Balance will show a Decrease of Mass, because the Gas is not Accounted for [So Closed System wouldn't work here] -You can then make a Graph, with the Change in Mass in the Y Axis, and Time in the X Axis