Lesson: Chapter - 5

Boyle’s Law

This is the first of the five-odd gas laws you’ll need to know for the SAT II Chemistry test. Study these laws closely because you are sure to see a question or two that asks you to apply them.

Boyle’s law simply states that the volume of a confined gas at a fixed temperature is inversely proportional to the pressure exerted on the gas. This can also be expressed as PV = a constant. This makes sense if you think of a balloon. When the pressure around a balloon increases, the volume of the balloon decreases, and likewise, when you decrease the pressure around a balloon, its volume will increase.

Boyle’s law to can also be expressed in the following way, and this is the form of the law that you should memorize:

P₁V₁ = P₂V₂

Example

Sulfur dioxide (SO₂) gas is a component of car exhaust and power plant discharge, and it plays a major role in the formation of acid rain. Consider a 3.0 L sample of gaseous SO₂ at a pressure of 1.0 atm. If the pressure is changed to 1.5 atm at a constant temperature, what will be the new volume of the gas?

Explanation

If P₁V₁ = P₂V₂, then (1.0 atm) (3.0 L) = (1.5 atm) (V₂), so V₂ = 2.0 L. This answer makes sense according to Boyle’s law—as the pressure of the system increases, the volume should decrease.

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