mol 5%-
- 4.6 X L
X7.96X105LX
6.02 X
loz3 S2- ions
1 mol S2-
ions
= 22 S2- ions
A precipitate will form upon the addition of the 23rd sulfide ion, a rather dramatic shift in the equilibrium as compared to the situation in 7.96 X lo5L of pure water where the number of S2- ions would be - 2.5 X 10-17 mol S2- ions X7.96X 1 0 5 L X 6.02 X loz3 S2- ions L 1mol S2- ions = 1.2 X loL3S2- ions
hangar. Initially, the volume of the air in the hangar must he converted to liters volume = 5.5 X 107ft3X ==
1.6 X
1 ft3
lo9L
Next, the mass of vanillin required must be determined mass =
2.0 X
lo-" L
g
X 1.6 X 109L
= 3.2 X 1 0 V g
Finally, the cost of the vanillin must be determined
What A Smell! Robert Perkins
Unlverslly 01 Brltlsh Columbla BC, Canada VBT 125
Vancouver,
The Guinness Book of World Records contains a wealth of information which has lwen used previously' to pmvide inrerrsrinr! vrot~lemsfor introductorv chemistry students. The fo~lowin~buestion is typical of some of the ones which I use with my students. Vanillin (4-hydroxy-3-methoxybenzaldehyde)is listed2 as the chemical whose aroma may he detected by the human nose at the lowest concentration. The threshold limit is 2.0 X 10-l1 g per liter of air. The Goodyear Airship hangar3 in Akron, Ohio, is the largest hangar in the world, having a volume of 5.5 X 10' f@. If the current price of 500 g of vanillin is $20.45,4 determine the cost to supply enough vanillin so that the aroma could he detectable anywhere in the Goodyear Airship -
- -
' Akers, H. A., Akers, G. E., J. CHEM.EDUC., 58, 795 (1981).
'
The Guinness Bookof WorldRecords. 1979 edition. p. 176. The Guinness Book of WorldRecords. p. 255.
Aldrlch Cafalog Handbook of Fine Chemicals. 1982-83, p. 1184.
384
Journal of Chemical Education
cost = 3.2 X
gX
$20.45 500 g
- $0.0013
The calculation has even more meaning if one brings in some authentic vanillin for the students to'smell. The sensitivity of the human nose can be discussed. The instructor can show the students just how small an amount 32 mg of vanillin corresnonds to and ask them to imaeine that small amount dispersed throughout the hangar. o n t h i s basis the class will likelssav that the nose is indeed sensitke todetect the vresence-of such a small amount of vanillin. However, if onethen calculates the concentration of vanillin molecules per liter of air 0.032 g 1 mol 6.02 X loz3molecules XX concentration = 1 mol 1.6 X 109L 152.2 g = 1.9 X 10lomolecules Thus the nose requires 79 billion molecules of vanillin per liter of air before it is able to detect them! The class is now in the position of trying to decide: is the nose sensitive or not? The author would like to thank a referee for helpful suggestions on this last section.
