Ideal Gas Equation
Chemistry: IB Diploma Higher Level, topic 1.5.1
The Ideal Gas Equation
Most gases obey a simple law known as the ideal gas equation. This describes the relationship between the pressure, volume and temperature of a gas.
The ideal gas equation
- The ideal gas equation is:
- Pressure × volume = number of moles × gas constant × temperature
- PV = nRT
- This equation can be used to calculate any variable in the equation, as long as you know the other three variables.
- P, V, n, and T are the variables.
- R is a constant - its value is always 0.08206.
Importance of units
- When using the ideal gas law, it is important to make sure you are using only SI units:
- Pressure must be in atmospheres (atm).
- Volume must be in liters (L).
- Temperature must be in Kelvin (K).
- These units are the scientific standard - they are all related and allow for easily understood numbers.
- Using different units will give you the wrong answer!
An example
- The pressure of 1 mole of a gas is 1 atm at a temperature of 298 K. What is its volume?
- PV = nRT
- V = nRT ÷ p
- V = 1 × 0.08206 × 298 K ÷ 1 atm
- V = 24.8 L
The Kelvin Scale
The Kelvin scale is a temperature scale that is commonly used in science. For the SAT Chemistry Subject Test you need to be able to convert °C to Kelvin.
The Kelvin scale
- The Kelvin scale is an absolute temperature scale defined to have 0 K at the lowest possible temperature, called absolute zero.
Absolute zero
- Absolute zero (0 K) is the lowest possible temperature.
- At absolute zero, there is no temperature.
- This means that there is no kinetic energy or vibration of molecules.
Converting from degrees to Kelvin
- To find degrees from Kelvin, just add 273.
- E.g. 0oC = 273 K.
- E.g. Water boils at 373 K.
Test yourself on Ideal Gas Equation
What is the ideal gas equation?
Which is true of the ideal gas equation?
Which of these is not a variable in the ideal gas equation?
How do particles move at absolute zero?
Which statements about absolute zero are correct?