Which equation expresses the Combined Gas Law describing the relationship of P, V, and T for a fixed amount of gas?

• A. P1V1/T1 = P2V2/T2
• B. PV=nRT
• C. P1V1 = P2V2
• D. V/T = constant

Answer: (A)

For a fixed amount of gas, the quantities P, V, and T are linked so that the product PV changes in proportion to T. Because n and R are constants, PV = nRT, which means PV/T is constant during any process of the same gas. Translating that into a two-state form, the relation becomes P1V1/T1 = P2V2/T2, showing how pressure, volume, and temperature transition from one state to another.

This form directly expresses the relationship for initial and final states, which is why it’s the best answer. The alternative PV/T = constant is another valid way to write the same idea but the two-state expression explicitly ties two states, making it the standard representation. The ideal gas law PV = nRT is a broader equation that includes n and R, not the two-state relationship for a fixed amount of gas. The form V/T = constant would ignore pressure and isn’t generally valid. The form P1V1 = P2V2 would imply temperature didn’t change, which isn’t required by the combined gas law.