Humboldt State University ® Department of Chemistry

Richard A. Paselk


General Chemistry

Fall 2009

Exercise: Gas Laws

© R. Paselk 2008

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Gas Law Problems

In these exercises we will be using the Ideal Gas Law or Perfect Gas Law equation:

PV = nRT

where R = the gas constant with units appropriate to the various measurements. We will use atm, L, K, and moles, so that R = 0.0821 (L*atm)/(mole*K)

I will base all of my examples on this equation because that requires a minimum of memorization. However you may find it easier to memorize a series of equations such as the "combined gas law equation"

1. Find the molar volume of a gas under standard conditions of temperature and pressure (STP).

2. A 1.00 L sample of gas weighed 1.25 g at a temperature of 0 °C and a pressure of 1.00 atm. What is the MW of this gas?

3. A rigid, sealed gas cylinder with a volume of 2.50 L is filled with hydrogen at a pressure of 820 mmHg and a temperature of 24 °C.

a. What will the pressure in the cylinder become if the temperature is raised to 450°C?

b. How many moles of hydrogen are there in this container?

4. A student ignores the warning labels and throws an empty (no liquid left, no spray) can of hair spray into his campfire.

a. Assuming an ambient temperature of 25 °C an atmospheric pressure of 7.20 x 102mmHg, and a temperature in the coals of 600 °C, find the pressure in the can in the fire, assuming it doesn't burst or expand (it is rigid).

b. If the volume of the can is 420 mL, how many moles of gas are in the can?

5. One of the student's colleagues on this ill fated trip tossed an "empty" 0.500 L propane cylinder into the fire. Unfortunately, 2.20 g of propane (C3H8) remained in the cylinder. What pressure would be reached in the cylinder assuming no deformation and no bursting at 6.00 x 102 °C.

6. 2.40 L of ethene gas (C2H4) is combined with 7.35 L of oxygen and ignited to give carbon dioxide as the only carbon containing product. If all volumes of reactants and products are measured at the same temperature and pressure (above 100 °C - so water is a vapor), calculate the volume of each substance after the reaction is complete.

7. Assume you have an engine with a 500 mL cylinder with a 10:1 compression ratio. If 0.200 L of methane and 0.300 L of oxygen are introduced into the cylinder at 765 mmHg and 25 °C, what will be the pressure "at the top of the stroke" if ignition gives a temperature of 557 °C?

Answers 1-7

In addition to these exercises you should familiarize yourself with the text materials.

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© R A Paselk

Last modified 6 November 2008