The ideal gas law, PV = nRT, is the most flexible form because it links pressure, volume, moles, and temperature in one equation. Use it when you know three of those variables and need the fourth. The combined gas law is better when the amount of gas stays the same but pressure, volume, and temperature change between two states. Boyle's law, Charles's law, and Gay-Lussac's law are simpler versions for cases where one variable is held constant.

Before solving, convert temperature to Kelvin. Gas volume and pressure are proportional to absolute temperature, not Celsius or Fahrenheit. A change from 10°C to 20°C is not a doubling of temperature because both values are far above absolute zero. In Kelvin, those temperatures are 283.15 K and 293.15 K. The difference is still 10 degrees, but the ratio used by the gas laws is based on the absolute values.

Pressure units must match the gas constant. If pressure is in atmospheres and volume is in liters, R = 0.08206 L atm per mol K works. If pressure is in pascals and volume is in cubic meters, R = 8.314 J per mol K works. Do not mix kPa, liters, and a gas constant chosen for a different unit system unless you convert first. Unit mismatches are more common than algebra mistakes in gas law work.

The amount of gas, n, is measured in moles. If a problem gives mass, divide by molar mass to get moles. For example, 32 g of oxygen gas O2 is 1 mol because the molar mass of O2 is about 32 g/mol. If the problem describes a sealed container and no gas enters or leaves, n stays constant and can often cancel out. If gas is added, removed, or reacts, the amount cannot be ignored.

Real gases depart from the ideal model when molecules are crowded or moving slowly. High pressure makes molecular volume matter. Low temperature makes attractions between molecules matter. Near room temperature and moderate pressure, many gases are close enough to ideal for classroom and first-pass engineering calculations. Near a phase change, in high-pressure cylinders, or in cryogenic systems, use real gas data or a correction model.

Gas laws are easier to remember when the particle picture is clear. Pressure comes from particles colliding with container walls. Raising temperature increases average kinetic energy, so particles hit harder and more often. Increasing volume gives particles more room, so wall collisions become less frequent. Adding more moles adds more particles, which increases pressure if volume and temperature stay fixed.

This particle view explains the direction of each law. Boyle's law says pressure rises when volume falls at constant temperature because the same particles collide with a smaller wall area more often. Charles's law says volume rises with temperature at constant pressure because a flexible container expands until collisions no longer create extra pressure. Gay-Lussac's law says pressure rises with temperature in a rigid container because the walls cannot move outward.