| Chem 107 |
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Fall 2008 |
| Lecture Notes: 6 November |
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| PREVIOUS |
Last time left off with Charles' Law. If we extrapolate the plot to zero volume we find the lowest temperature possible, or absolute zero = 0 K.
The intercept on the volume axis is then taken as absolute zero = -273.15 °C = 0 K for an ideal or "perfect" gas with particles of zero volume and no interactions other than collisions.
Algebraically we then find that V = k'T, & & V1/T1 = V2/T2.
We can combine these relationships (T was part of the constant for Boyle's Law and P is part of the constant for Charles' Law) to give"
where R = the gas constant with units appropriate to the various measurements. We will use atm, L, K, and moles.
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" on pg 356 of your text etc.
As an example, let's find the molar volume of a gas under standard conditions of temperature and pressure (STP). STP are defined as: P = 1 atm and T = 0° C. Thus we need to solve the gas equation for 1 mole of gas at 273.15 K (= 0°C+ 273.15) and 1 atm:
Let's look at some sample problems.
Example: We can find the MW of a gas given its density. Thus a 1.000 L sample of gas weighed 1.25 g at a temperature of 0.0 °C and a pressure of 1.000 atm. What is the MW of this gas?
Example: A student ignores the warning labels and throws an empty (no liquid left, no spray) can of hair spray into his campfire. Assuming an ambient temperature of 25 °C and 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.
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© R A Paselk
Last modified 6 November 2008