Final answer:
The amount of gas must remain constant to use the combined gas law for calculations.
Step-by-step explanation:
The combined gas law is used to calculate the relationship between the pressure, volume, and temperature of a gas when all three variables change. In order to use the combined gas law for calculations, the amount of gas must remain constant. The combined gas law equation is expressed as:
P₁V₁/T₁ = P₂V₂/T₂