Chemistry Reference
In-Depth Information
Here we have a student who collects a 2.30 dm
3
sample of carbon diox-
ide with a pressure of 92.7 kPa and a temperature of 17.0
o
C. If he wants to
determine the number of molecules of carbon dioxide that his sample con-
tains, what would he need to do?
First, he would have to convert
o
C to Kelvin, because the gas law calcu-
lations aren't based on the Celsius scale.
Step 1—Converting Celsius to Kelvin.
K =
o
C + 273 = 17.0
o
C + 273 = 290 K
Next, he would use the Combined Gas Law (review Lesson 8-5) to
adjust the volume of his gas to standard temperature and pressure.
Step 2—Adjusting the volume of the gas at room temperature to STP.
V
1
P
1
T
2
P
2
T
1
(2.30 dm
3
)(92.7 kPa)(273 K)
(101.3 kPa)(290 K)
V
2
=
=
= 1.98 dm
3
Now, he would use the molar volume of a gas to determine the
number of moles of gas he collected.
Step 3—Determining the number of moles represented by 1.98 dm
3
of a
gas at STP.
# of moles =
volume of gas sample at STP
molar volume of gas at STP
1.98 dm
3
22.4 dm
3
/mole
=
= 0.088 moles
If you still remember the original question, he wanted to know how
many molecules of carbon dioxide he generated. Therefore, his last
move would be to convert moles to number of particles, by multiplying
by Avogadro's number.
Step 4—Determining the number of molecules in 0.088 moles.
# of molecules = 0.088 moles ×
6.02 × 10
23
molecules
mole
= 5.3 × 10
22
molecules
And we have our answer. Imagine that! Chemistry gives us the ability
to collect an invisible gas and count the microscopic particles that are too
small for us to even see!
Perhaps you thought of another way to solve the same problem, using
one less step. One of the most enjoyable things about problem solving in
the sciences is that there are often a number of paths and formulas that
you can follow and still get to the correct answer. If you are the type of
person who enjoys problem solving, then getting there is half the fun.
Let's take the same original problem, but solve it using the Ideal Gas Law.
