ACT Science: Ideal Gas Law—PV=nRT Connects Pressure, Volume, Temperature

Ideal Gas Law: PV=nRT, where P=pressure, V=volume, n=moles of gas, R=gas constant (0.082 L·atm/mol·K), T=absolute temperature (Kelvin). This equation shows relationships: If you increase T (heat), pressure increases (if volume is constant). If you increase P (compress), volume decreases. This single equation predicts how gases respond to changes in conditions. Most ACT gas questions ask: "If T doubles, what happens to P?" or "If V decreases, what happens to P?" Use the law to reason through it.

Key insight: Ideal gases follow this law. Real gases deviate slightly at extreme pressures/temperatures, but ACT assumes ideal behavior.

Scenario 1 (Temperature increase): A balloon contains 1 mole of gas at 300 K and 1 atm pressure, volume 24.6 L. You heat it to 600 K. New volume? P and n are constant. V/T=constant → V1/T1=V2/T2 → 24.6/300=V2/600 → V2=49.2 L. Volume doubles when T doubles (in Kelvin). Scenario 2 (Pressure change): You compress a gas from 2 L to 1 L at constant T. Pressure increases. P1V1=P2V2 → P1(2)=P2(1) → P2=2P1. Pressure doubles. Scenario 3 (Mixed change): A gas at 1 atm, 1 L, 300 K is heated to 600 K and compressed to 0.5 L. New pressure? PV/T=constant → (1)(1)/300=(P2)(0.5)/600 → P2=(1)(600)/(300)(0.5)=4 atm. Apply PV/T=constant for problems with multiple changing variables.

Always convert temperature to Kelvin (add 273 to Celsius).

Problem 1: A 10-L container holds 2 moles of gas at 300 K. Pressure? PV=nRT → P(10)=2(0.082)(300) → P=4.92 atm. Problem 2: Volume is halved at constant T and n. How does pressure change? P1V1=P2V2 → P1(V)=P2(0.5V) → P2=2P1. Pressure doubles. Problem 3: A gas at 1 atm, 10 L, 300 K is heated to 600 K and volume stays 10 L. New pressure? P1/T1=P2/T2 → 1/300=P2/600 → P2=2 atm. Complete all three daily until gas law calculations are fast.

Verify: If T doubles and V is constant, P doubles. If V halves and T is constant, P doubles. ✓

Ideal gas law questions appear in 1-2 ACT Science passages. They are computational once you know PV=nRT and its rearrangements. Investing 20 minutes in gas law yields 1-2 guaranteed points because the equation is simple and behavior is predictable.

Master the ideal gas law two days before the test. By test day, gas problems become routine.