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IDEAL AND REAL GAS LAWS Gases, unlike solids and liquids have indefinite shape and indefinite volume. As a result, they are subject to pressure changes, volume changes and temperature changes. Real gas behavior is actually complex. For now, let's look at ideal Gases, since their behavior is simpler. By understanding ideal gas behavior, real gas behavior becomes more tangible. How do we describe an ideal gas? An ideal gas has the following properties: 1. An ideal gas is considered to be a "point mass". A point mass is a particle so small, its mass is very nearly zero. This means an ideal gas particle has virtually no volume. 2. Collisions between ideal Gases are "elastic". This means that no attractive or repulsive forces are involved during collisions. Also, the kinetic energy of the gas molecules remains constant since theses interparticle forces are lacking. Volume and temperature are by now familiar concepts. Pressure, however, may need some explanation. Pressure is defined as a force per area. When gas molecules collide with the sides of a container, they are exerting a force over that area of the container. This gives rise to the pressure inside the container. Problems Dealing With The Ideal Gas Law For a gas, pressure, volume, temperature and the moles of gas are all related by the following equation: The units of pressure, volume and temperature are dictated by the ideal gas law constant, R. The most common used value for R when dealing with gases is 0.0821 L . atm/mol . K. This unit requires that volume to be expressed in liters, pressure to be expressed in atmospheres, and temperature to be expressed in Kelvin. One thing to keep in mind is that temperature will always be expressed in the Kelvin scale when dealing with any of the gas laws. Let's start looking at some of the types of questions you may encounter using the ideal gas law. In some problems, you will know four out of the five possible variables, and be asked to solve for the fifth variable. It is important to note that the pressure, volume, moles or temperature of the gas is not changed. When these
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variables are changed, a different type of problem is apparent, and we will shortly look at these types of problems. Suppose you have 1.00 mol of a gas at 0 o C, occupying a container which is 500 mL in size. What is the pressure of this gas in atmospheres? To solve this problem, consider that moles, temperature, volume and the ideal gas law constant, R, are known. Pressure is the only unknown variable. Recall that R will dictate the units. The temperature is given in Celsius. This must be converted to the Kelvin scale. To convert Celsius to Kelvin, add 273 to the Celsius temperature: K = o C + 273 = 0 o C + 273 = 273 K Also, the volume must be in liters, not milliliters. Convert as follows: Now we are ready to insert the known values into the ideal gas law: Solve for pressure by dividing both sides of the equation by the volume, 0.500 L: Notice how the units cancel to give atmospheres. Next, let's find the volume of 2.50 mol of gas which is at 730 mm Hg of pressure,
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This note was uploaded on 03/13/2008 for the course PHY 122 taught by Professor Svetlana during the Spring '07 term at Cal Poly Pomona.

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