Page 27 - A Working Method Approach For Introductory Physical Chemistry Calculations

Introduction to Physical Chemistry 11
3. Write down the balanced chemical equation if necessary, in-
cluding the stoichiometry factors, VA, vB, vc and VD respectively:
VAA + VBB+VCC + vDD.
4. Identify all the data given in the question, including any
constants which are involved, e.g. R = 8.314 J K-' mol-', 1 F
= 96500 C mol-' (defined later), 1 mole of a gas behaving
ideally at 25 "C and 1 bar pressure occupies 24.8 dm3.
5. Convert all units to the one system, i.e. change "C to K, hours
to seconds, etc. Watch out especially for (a) standard state
conditions, e.g. E", AH", AS" and AGO parameters: 25°C
(298 K) and 1 bar pressure, and; (b) R = 0.08314 dm3 bar K-'
mol-' (used when the pressure is expressed in bar) or more
generally R = 8.314 J K-' mol-'. For example, suppose you
were asked to calculate the volume occupied by 0.5 mol of N2(g)
at 280 K and 0.93 bar, using the ideal gas equation, i.e. pV =
nRT, and rearranging, V = nRT/p. Since p, the pressure of the
gas, is given in bar, R = 0.08314 dm3 bar K-' mol-' must be
used, not R = 8.314 J K-' mol-'. Therefore, V = (0.5
mol) x (0.08314 dm3 bar K-' mol- ') x (280 K)/0.93 bar =
12.52 dm3.
6. Identify the unknown in the question, i.e. what quantity is being
looked for specifically? Do not be afraid to sketch a simple
diagram if this identifies the problem for you!
7. Write down all relevantformulae. The question will most likely
involve just one or, less likely, more than one, of these equa-
tions.
8. Substitute the known data into the equations in step 7 above
(this may help to identify the unknown) and solve for the
unknown. Remember, it is easier to rearrange the equations
before substituting the numerical values.
9. Write down your answer (never have 'x = 0' your final
as
answer) and ensure that the appropriate units are given. Re-
member, logarithmic values are dimensionless, i.e. they have no
units. For example, suppose that the concentration of H30+
ions in a solution is 0.04 M. Then, the pH (defined in Chapter 5)
can be calculated using the standard equation, pH
= -loglo[H30+] = -log10(0.04 M) = 1.40, i.e. although the
units of concentration are mol dm-3, there are no units for pH,
since logs are involved.
10. Re-read the question and answer any riders to the question!

Armed with such an approach, most numerical problems in intro-