Beginner Topic 231 of 1102 States of Matter & Solutions
Combined Gas Law
Combines Boyle, Charles and Gay-Lussac laws: P1V1/T1 equals P2V2/T2.
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In-depth explanation of Combined Gas Law
The combined gas law merges Boyle's, Charles's and Gay-Lussac's laws into one expression, P1V1/T1 = P2V2/T2. It applies to a fixed amount of gas whose pressure, volume and temperature all change between two states. Each of the earlier laws is recovered by holding one variable constant, and temperature must always be in kelvin. To use the law, identify the initial and final conditions, rearrange for the unknown quantity, and check that the result is physically sensible. It is especially useful in problems where pressure, volume and temperature all vary at the same time.
Going deeper: Combined Gas Law
Key ideas: Solve by writing the known and unknown values for states 1 and 2, then rearranging, for example V2 = V1 (P1/P2)(T2/T1). Each factor can be checked separately: higher pressure shrinks volume, higher temperature expands it. Pressure units need to match only between P1 and P2, and temperature must be kelvin.
Common mistakes and connections: Common errors include inverting one ratio, forgetting kelvin, and applying it when the amount of gas changes, such as with a leaking container. If moles change, use PV = nRT or add n to the ratio. It reduces to Boyle's, Charles's or Gay-Lussac's law when one variable is fixed, and to PV/nT = R.
Visualization of Combined Gas Law
The combined gas law P1V1/T1 = P2V2/T2: curves of constant temperature on a P-V graph show how changing T shifts them for a fixed amount of gas. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A gas at 1 atm, 2 L and 300 K is compressed to 4 atm and heated to 600 K, giving a final volume of 1 L.
Frequently asked questions about Combined Gas Law
What is Combined Gas Law?
Combines Boyle, Charles and Gay-Lussac laws: P1V1/T1 equals P2V2/T2.
Is Combined Gas Law a beginner, intermediate or advanced topic?
Combined Gas Law is a beginner topic in States of Matter & Solutions. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Combined Gas Law?
Study these earlier topics in States of Matter & Solutions first: Charles's Law, Gay-Lussac's Law, Avogadro's Law.
What comes after Combined Gas Law?
The next topics in States of Matter & Solutions are Ideal Gas Law and Gas Constant.
Can I practise Combined Gas Law with quizzes?
Yes. The interactive lesson includes a self-check question on Combined Gas Law, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Combined Gas Law fits in States of Matter & Solutions
Combined Gas Law is topic 12 of 109 in States of Matter & Solutions.