Gas Law Relationships for AP Chemistry
Practice problems for these concepts can be found at:
- Gases Multiple Choice Review Questions for AP Chemistry
- Gases Free-Response Questions for AP Chemistry
When we use the word pressure, we may be referring to the pressure of a gas inside a container or to atmospheric pressure, the pressure due to the weight of the atmosphere above us. These two different types of pressure are measured in slightly different ways. Atmospheric pressure is measured using a barometer (Figure 8.1).
An evacuated hollow tube sealed at one end is filled with mercury, and then the open end is immersed in a pool of mercury. Gravity will tend to pull the liquid level inside thetube down, while the weight of the atmospheric gases on the surface of the mercury pool will tend to force the liquid up into the tube. These two opposing forces will quickly balance each other, and the column of mercury inside the tube will stabilize. The height of the column of mercury above the surface of the mercury pool is called the atmospheric pressure. At sea level the column averages 760 mm high. This pressure is also called 1 atmosphere (atm). Commonly, the unit torr is used for pressure, where 1 torr = 1 mm Hg, so that atmospheric pressure at sea level equals 760 torr. The SI unit of pressure is the pascal (Pa), so that 1 atm = 760 mm Hg = 760 torr = 101,325 Pa (101.325 kPa). In the United States pounds per square inch (psi) is sometimes used, so that 1 atm = 14.69 psi.
To measure the gas pressure inside a container, a manometer (Figure 8.2) is used. As in the barometer, the pressure of the gas is balanced against a column of mercury.
Ideal Gas Equation
The ideal gas equation has the mathematical form of PV = nRT, where:
- P = pressure of the gas in atm, torr, mm Hg, Pa, etc.
- V = volume of the gas in L, mL, etc.
- n = number of moles of gas
- R = ideal gas constant: 0.0821 L·atm/K·mol
- T = Kelvin temperature
This is the value for R if the volume is expressed in liters, the pressure in atmospheres, and the temperature in kelvin (naturally). You could calculate another ideal gas constant based on different units of pressure and volume, but the simplest thing to do is to use the 0.0821 and convert the given volume to liters and the pressure to atm. And remember that you must express the temperature in kelvin.
Let's see how we might use the ideal gas equation. Suppose you want to know what volume 20.0 g of hydrogen gas would occupy at 27°C and 0.950 atm. You have the pressure in atm, you can get the temperature in kelvin (27°C + 273 = 300.K), but you will need to convert the grams of hydrogen gas to moles of hydrogen gas before you can use the ideal gas equation. Also, remember that hydrogen gas is diatomic, H2.
First you'll convert the 20.0 g to moles:
- (20.0 g/l) × (1 mol H2/2.016 g) = 9.921 mol H2
(We're not worried about significant figures at this point, since this is an intermediate calculation.)
Now you can solve the ideal gas equation for the unknown quantity, the volume.
- PV = nRT
- V = nRT/P
Finally, plug in the numerical values for the different known quantities:
- V = (9.921 mol)(0.0821 L atm/K mol) (300.K)/0.950 atm
- V = 257 L
Is the answer reasonable? You have almost 10 mol of gas. It would occupy about 224 L at STP (10 mol × 22.4 L/mol) by Avogadro's relationship. The pressure is slightly less than standard pressure of 1 atm, which would tend to increase the volume (Boyle's law), and temperature is greater than standard temperature of 0°C, which would also increase the volume (Charles's law). So you might expect a volume greater than 224 L, and that is exactly what you found.
Remember, the final thing you do when working any type of chemistry problem is answer the question: Is the answer reasonable?
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