a
SOLUTION OF PROBLEMS IN CHEMISTRY EVERETT A. TROUSDALE Naselle-Grays River Valley High School, NaseUe, Washington
TREmole is the concept of quantity which is of unique importance t o chemists. There is great pedagogic value to introducing students t o its importance and utility when they first begin to solve quantitative chemical problems. It has been the author's experience that the system here suggested is successful with high school students, even those whose earlier experience with applied arithmetic had made little lasting impression. It has been apparent that these beginning chemistry students have gained more real understanding of chemical relationships than those who have used exclusively the 'Lproportion" methods presented in many high school texts. When a chemical equation is properly balanced, the coefficients of the formulas give the relative number of moles involved. When the data for the known are suplied in terms of grams, liters of gas (S.T.P.) or number of molecules, the first step is to calculate the number of moles so represented. Since the mole is defined as a gram molecular weight which occupies 22.4 liters (S.T.P.), if a gas, and contains 6.023 X lozamolecules, it is usually most convenient to translate other weight units into grams and to convert gas volumes from other conditions to S.T.P. It is of course possible to calculate the number of molecules in a "pound mole" or the corresponding volume of a gas in any other appropriate units or a t any other set of conditions. A combining ratio can be defined as the number of Quantity of known
Unknown (required)
Moles of known
moles of unknown divided by the number of moles of known as shown by the coefficients in the equation. The solution to any problem is then given in terms of moles of unknown if the actual number of moles of known is multiplied by this combining ratio. The answer can be converted t o grams, liters (S.T.P.) of a gas or numher of molecules, as may be required by the statement of the problem, by multiplying the answer in moles by the appropriate conversion factor. Since the known may be given in grams, liters (S.T.P.), or molecules and the answer required in any one of the three same units, there are nine variations of the problem. The nine variations of the problem are illustrated in the followingtable using the equation Moles
-
1 Bigs,
8 + 8HNOJ
-
-
+
com-
Moles of unknoum
bining ratio
Quantitg qf unknown
(Note: Wherever the word "liters" is used. S.T.P. is understood) 590 3 590 3 295 2 295 2 2 189 2 189 liters NO ex?i 63 8 59 4 59 4 molecules HsO 5Bx2 295 2
11.2 liters NO
grams Bi(N0a)s
11.2 22.4
2 2
11.2 2 2
T
44.8 liters H*0
liters NO
44.8 22.4
2 4
44.8 5 4
7
5 . 6 liters NO
molecules H90
5.6 22.4
-
4 2
5.6 ~
4 4
1.506 X loP" molecules S
grams H 2 0
1.506 X loZS 6.023 X loZS
4 3
1.506 X 6.023 X
liters NO
6.023 X 6.023 x
2 8
6.023 X 10" 6.023 x 10"
molecules Bi&
1.205 X 10" 6.023 X los8
grams S
-
loP'
loZS
-
2
NO. 6, JUNE, 1958
(g
4 X 6.023 X 10") X molecules H 2 0 2 (11.2 22.4 x x 295) grams Bi(NOdr 44.8 x 2 x 22.4) liters
,
,(
2
H*O ~
lozP 4 loaJ 3 2
8
2 1
Bi(NO& Molecular weights: BiBn. . ,514, HNOJ. . .63, Bi(N03)s.. ,295, H20.. .IS, NO. . .30, S. . .32. 35,
2 4 + 3S3 + 2NO + 4HzO
For some classes it might be better to make up a table from a simpler reaction such as 2C0 O1 2C02, using for example two moles of CO for the production of 2 moles of COz and using the same amount of CO in each case. Thus starting with grams, liters or molecules in the lst, 4th, and 7th variations the answer in grams would be the same. Similarly the same answer would appear in other parts of the chart. The author would like to acknowledge his indehtedness to Professor Paul Urone of the University of Colorado for the basic method.
590 grams Bi(NOs)r 189 grams HNOI 59 grams Bi(NO&
6.023 X loP4 molecules HNO* 1.205 X 10" moleoules
VOLUME
2 ZBi(NO&
X 6.023 X loss (& 2 X molecules H.0
(1.506 X loz3 6.023 X loza grams H 2 0 6.023 X loZ4 (6.023 x loPJ liters NO
+ 2
8
22'4)
/
299
