Page 28 - A Working Method Approach For Introductory Physical Chemistry Calculations

12 Chapter I
ductory physical chemistry can be broken down and tackled, no
matter how difficult they may first appear.

Worked Example

Example: A gas G, occupying 2.5 dm3 at 0 "C and 1 bar pressure, is
transferred to a 5.5 dm3 container, where the pressure is 0.25 bar.
In order for the gas in the new vessel to attain this pressure, what
must the temperature of the gas be?

Solution:

1. Read the question carefully-looks complicated. . . ? Just break
it down!
2. No balanced chemical equation is involved, so step 2 can be
skipped in this case.
3. One species involved-a gas G(gI!
4. Identify the data in the question: V1 = 2.5 dm3; TI = 0 "C; p1 =
1 bar; V2 = 5.5 dm3;p2 = 0.25 bar.
5. Convert temperature to kelvins: Tl = (0 + 273) K = 273 K.
6. Unknown = TZ!
7. Pressure, volume and temperature suggest that the required
equation is: (plVl)/Tl = (p2V2)/T2, i.e. 'Peas and Vegetables go
on the Table'!
8. Rearrange the equation before substituting the numerical values:
T2 = (p2/p1)(V2/V1)(T1). Solve for T2, i.e. T2 = [0.25 bar/l
bar] x [5.5 dm3/2.5 dm3] x [273 K] = 150.15 K.
9. Answer: Temperature is 150.15 K. Notice how the units cancelled
each other conveniently in step 8. In this question the temperature
could have been left in "C, but it is good practice always to
convert temperature to the absolute temperature, measured in K.

WORKING METHOD FOR GRAPHICAL PROBLEMS
A corresponding working method can be applied to graphical pro-
blems in physical chemistry, adopting the same approach.

1. Read the question carefully.
2. Identify the tabulated data given to you; tables of data normally
mean a graph has to be plotted. Remember, you might not
necessarily be told this in a problem.