Chemistry Reference
In-Depth Information
This path is actually shorter and easier, but for a beginner, it may be a little
harder to understand when you see it at first. If it seems confusing, just
slow down and read it a few times. We will be using the Ideal Gas Law from
Lesson 8-7, so you may want to review it.
The original problem was this: A student collected a 2.30 dm
3
sample
of carbon dioxide 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 contains, what would he need to do?
The first step is going to be the same, because he needs to convert
o
C to
Kelvin.
Step 1—Converting Celsius to Kelvin.
K =
o
C + 273 = 17.0
o
C + 273 = 290 K
Now, instead of using the Combined Gas Law, he can use the Ideal
Gas Law to find out how many moles of carbon dioxide he has.
Step 2—Determining the number of moles of carbon dioxide he has.
Given:
Pressure (P) = 92.7 kPa Volume (V) = 2.30 dm
3
Temperature (T) = 290 K Constant (R)
= 8.31 dm
3
× kPa/moles × K
Find:
Number of moles (n)
Formula: PV = nRT
PV
RT
nRT
RT
PV
RT
Isolate:
so,
=
n =
PV
RT
(92.7 kPa)(2.30 dm
3
)
(8.31 dm
3
× kPa/moles × K)(290 K)
Solve:
n =
=
= 0.088 moles
Notice that our answer for the number of moles is the same as the
answer that we got after Step 3, using the previous method. We still
have one more step, and that is to convert the number of moles to the
number of atoms of carbon dioxide. Again, this step will be identical to
Step 4 in our previous method.
Step 3—Determining the number of molecules in 0.088 moles.
6.02 × 10
23
molecules
mole
# of molecules = 0.088 moles ×
= 5.3 × 10
22
molecules
And we arrive at the same answer as last time, only this time we did
it with one less step.
