Ideal Gases
You will definitely see some questions on gases and the laws that govern them on the SAT II Chemistry exam. All of the gas laws rely on some basic assumptions that are made about gases, and together they constitute what it means for a gas to be in an ideal state. In an ideal state
- All gas particles are in constant, random motion.
- All collisions between gas particles are perfectly elastic (meaning that the kinetic energy of the system is conserved).
- The volume of the gas molecules in a gas is negligible.
- Gases have no intermolecular attractive or repulsive forces.
- The average kinetic energy of the gas is directly proportional to its Kelvin temperature and is the same for all gases at a specified temperature.
Only four measurable properties are used to describe a gas: its quantity, temperature, volume, and pressure. The quantity (amount) of the gas is usually expressed in moles (n). The temperature, T, of gases must always be converted to the Kelvin temperature scale (the absolute temperature scale). The volume, V, of a gas is usually given in liters. Finally, the pressure, P, of a gas is usually expressed in atmospheres. Gases are often discussed in terms of standard temperature and pressure (STP), which means 273K (or 0ºC) and 1 atm.
Example
Which of the following statements is not true of ideal gases?
- The volume occupied by gas particles is only significant at very low pressures.
- Gas molecules occupy an insignificant volume compared to the volume of the container that holds them.
- The particles of a gas move in random straight line paths until a collision occurs.
- The collisions that occur between gas particles are considered elastic.
- At a given temperature, all gas molecules within a sample possess the same average kinetic energy.
Explanation
In this example, choice 1 is incorrect. Choices 2, 3, 4, and 5 all describe an ideal gas. Choice 1 makes an incorrect assumption: it begins with a true statement about volume not being very significant but then turns around and gives the incorrect scenario—if the pressure is low, then gas particles undergo very few collisions, so the volume is insignificant. The volume only becomes significant if gas particles collide often, increasing the chances that intermolecular forces will hold them together.
Measuring the Pressure of a Gas
Gas pressure is a gauge of the number and force of collisions between gas particles and the walls of the container that holds them. The SI unit for pressure is the pascal (Pa), but other pressure terms include atmospheres (atms), millimeters of mercury (mmHg), and torr. The following is a list of all of the standard pressure in every unit for pressure. Memorize these for the exam so you can convert units where necessary:
