Gas laws describe how pressure, volume, temperature, and amount of gas are related. This cheat sheet helps students choose the correct equation, identify units, and solve common gas law problems. It is useful for chemistry labs, homework, and test review because gas variables often change together.
The most important idea is that gases are modeled using proportional relationships. Boyle's law connects pressure and volume, Charles's law connects volume and temperature, and Gay-Lussac's law connects pressure and temperature. The combined gas law and ideal gas law bring several variables together, while Dalton's law explains mixtures of gases.
Key Facts
- Boyle's law states that pressure and volume are inversely proportional when temperature and moles stay constant: .
- Charles's law states that volume and temperature are directly proportional when pressure and moles stay constant: .
- Gay-Lussac's law states that pressure and temperature are directly proportional when volume and moles stay constant: .
- The combined gas law relates pressure, volume, and temperature for a fixed amount of gas: .
- The ideal gas law relates pressure, volume, moles, and temperature: .
- Temperature in gas law calculations must be measured in kelvins, using .
- Dalton's law of partial pressures states that the total pressure of a gas mixture is the sum of each gas pressure: .
- At standard temperature and pressure, mole of an ideal gas has a volume of about at and .
Vocabulary
- Pressure
- Pressure is the force of gas particle collisions per unit area, often measured in , , or .
- Volume
- Volume is the amount of space a gas occupies, usually measured in or .
- Kelvin
- Kelvin is the absolute temperature scale used in gas laws, where represents absolute zero.
- Mole
- A mole is an amount of substance equal to particles.
- Ideal Gas
- An ideal gas is a model gas whose particles have no volume and no intermolecular attractions.
- Partial Pressure
- Partial pressure is the pressure one gas in a mixture would exert if it occupied the container alone.
Common Mistakes to Avoid
- Using Celsius in gas law equations is wrong because proportional gas laws require absolute temperature. Always convert with before substituting.
- Mixing pressure units is wrong because equations compare or combine pressures directly. Convert all pressures to the same unit, such as or , before solving.
- Choosing Boyle's law for a temperature change is wrong because Boyle's law assumes temperature is constant. If pressure, volume, and temperature all change, use .
- Forgetting that inverse relationships move in opposite directions is wrong for Boyle's law. If volume decreases at constant temperature, pressure must increase.
- Using the wrong gas constant is wrong because must match the pressure and volume units. For example, use when pressure is in and volume is in .
Practice Questions
- 1 A gas has a volume of at . If the pressure increases to at constant temperature, what is the new volume?
- 2 A balloon contains of gas at . What volume will it have at if pressure stays constant?
- 3 How many moles of gas are in a container at and ? Use .
- 4 Explain why a sealed aerosol can may become dangerous when heated, using particle motion, pressure, and temperature.
Understanding Gas Laws
Gas laws make sense when viewed through particle motion. Gas particles move quickly in random directions and collide with the walls of their container. Each collision pushes on a wall.
The combined effect of countless collisions creates pressure. Heating a gas gives its particles more average kinetic energy. They move faster and hit walls more often and with greater force.
If the container can expand, its volume grows. If it is rigid, the pressure rises instead. This particle model helps students predict a change before choosing any equation.
Temperature needs special care because Celsius zero does not mean particles have no motion. Gas law relationships depend on absolute temperature, measured in kelvins. A change from ten degrees Celsius to twenty degrees Celsius is not a doubling of temperature.
In kelvins, it changes from about two hundred eighty three to two hundred ninety three, which is only a small increase. Convert Celsius to kelvins before placing a temperature into a calculation.
Keep extra digits during work, then round the final answer to match the data given. A negative Celsius temperature can still represent a positive Kelvin temperature.
Units are another common source of errors. Pressure may be reported in atmospheres, kilopascals, millimeters of mercury, or torr. Volume may be in liters or milliliters.
The gas constant used in an ideal gas calculation must match the chosen pressure and volume units. For example, a value of the gas constant designed for liters and atmospheres cannot be used unchanged with kilopascals. Write units beside every number while solving.
Canceling units as you work can reveal a mismatch early. In two state problems, use the same unit for each pressure and each volume. Conversion is often needed before substitution.
Real gases do not behave perfectly under every condition. The ideal model assumes particles take up almost no space and have no attractive forces. This is a good approximation at low pressure and moderate or high temperature, where particles are far apart.
At high pressure, particle volume becomes important. At low temperature, attractions can pull particles closer and may lead to condensation. Gas laws appear in bicycle pumps, aerosol cans, weather balloons, lungs, car tires, and pressure cookers.
In laboratory work, a gas collected over water contains water vapor. The measured pressure includes the vapor pressure of water, so that contribution must be removed before finding the pressure of the dry gas.
For mixtures such as air, each gas contributes its own share of the total pressure. This explains why oxygen can have a lower partial pressure at high altitude even though air still fills the atmosphere.