Chemistry Reference
In-Depth Information
Hopefully, Example 2 shows you how the value for the constant, and
the units that come with the constant, can be determined.
Suppose we collected a sample of chlorine gas in the laboratory, which
was not at STP, and we wanted to determine the number of moles that the
sample contained. With the calculations that we learned in earlier lessons,
we would be able to adjust the volume of the gas to STP, using the Com-
bined Gas Law. Then, we could use the known molar volume of a gas to
determine the number of moles of chlorine gas we had. The Ideal Gas Law
allows us to do this work in a single calculation, as shown in Example 3.
Example 3
A student collects a sample of chlorine gas, which occupies 0.750 dm
3
at a temperature of 297 K and a pressure of 103.2 kPa. How many
moles of chlorine does this sample represent?
Given: V = 0.750 dm
3
; T = 297 K; P = 103.2 kPa;
R = 8.31 dm
3
× kPa/mol × K
Find:
n
PV
RT
(103.2 kPa)(0.750 dm
3
)
(8.31 dm
3
× kPa/mol × K)(297 K)
Formula:
n
=
=
= 0.0314 moles
Just as a reminder, for our final problem let's try an example where
you are required to make a couple of conversions before you can solve the
problem. You will want to remember the formulas for temperature and
pressure conversions, which we covered in earlier chapters.
Example 4
How much space would 3.45 moles of nitrogen gas occupy at a
temperature of 18
o
C and a pressure of 888 mm of Hg?
I believe in getting in the habit of making the necessary conversions as
early as possible. I often make the conversions, and then cross out the origi-
nal values and write the converted values right in the problem.
We would convert the temperature to Kelvin with this formula:
K =
o
C + 273 = 18
o
C + 273 = 291 K
