The volume occupied by an ideal gas at constant temperature and pressure is proportional to the number of moles present. Which law is this?

• A. Avogadro's Law
• B. Gay-Lussac's Law
• C. Dalton's Law
• D. Kelvin's Law

Answer: (A)

When temperature and pressure are fixed, the volume required by a gas scales directly with how many moles of gas are present. This is Avogadro's law: equal volumes at the same temperature and pressure contain the same number of moles. In terms of the ideal gas equation PV = nRT, holding T and P constant makes V proportional to n, so doubling the amount of substance doubles the volume. This intuitive idea—more particles need more space while the conditions stay the same—is why Avogadro's law is the correct description here. The other laws relate changes between two variables for a fixed amount of gas, not volume changing with the amount of gas at fixed T and P.