Untitled - Kelly Walsh High School

Gases 81
Quick Tip
Don’t Forget!
moles of gas) constant, that is, no gas can get in or out; we can express the relationship
between the other three by the combined gas equation:
P 1 V 1
T 1
P 2 V 2
T 2
1 and 2 refer to the initial and final conditions of the gas, respectively.
In any gas law calculation, you must express the temperature in Kelvin.
For example, suppose you have a balloon occupying a volume of 10.0 L at an
internal pressure of 1.50 atm at 20. o C. It is heated to 70. o C and the pressure is
determined to be 2.00 atm. We can calculate the new volume using the combined
gas law.
We will be solving the combined gas equation for V 2, so we will take the combined
gas law and rearrange for V 2:
V 2 P 1 V 1 T 2
T 1 P 2
The temperatures must be expressed in Kelvin. 20.C 293 K (K C 273)
and 70.C 343 K.
Substituting in the values:
V 2
(1.50 atm) (10.0 L) (343 K)
(293 K) (2.00 atm)
8.77986 8.78 L
In the combined gas equation, we held just the amount constant. If, however,
we hold two quantities constant and look at the relationship between the other
two we can derive the other common gas laws shown below in Table 5-1.