48 QUANTITATIVE ASPECTS OF CHEMISTRYAvogadro's Number. The actual number of molecules of a gas
in 22.4 liters under standard conditions has been determined by
several different methods. The results agree very well, and the
value generally accepted is 6.06 X 10^23. This quantity has been
named Avogadro's number in honor of the man who first suggested
the principle on which it depends. It is the number of molecules
in a mole of any substance, whether gas, liquid, or solid. It is
also the number of atoms in a gram atomic weight of an element.
Atomic Weights. In an earlier section (page 39) it was shown
that the atomic weight of an element could be obtained by multi-
plying the combining ratio of the element with oxygen by 16, the
atomic weight of oxygen, and dividing the result by the assumed
atomic ratio. We are now in a position to verify the atomic
weights so found and thus to prove whether the assumed atomic
ratio was correct.
We shall start with the molecular weight of oxygen as 32 and
first demonstrate that on this basis the atomic weight of oxygen
is 16.
SubstanceWater
Carbon dioxide
Sulphur trioxide
Acetic acid
Phenol
Nickel carbonyl
Phosphorus oxy-
chloride
OxygenWeight in
grams of 1
mole18 016
44
80
60
94
170 8153 4
32Percentage
by weight
of oxygen88 81
72 7
60
53 3
17 02
37 510 43
100Weight of
oxygen in
1 mole16
32
48
32
16
6416
32Greatest
common
divisor1 X 16
2X 16
3X 16
2 X 16
1 X 16
4X 161 X 16
2 X 16There are hundreds of gaseous compounds of oxygen whose
densities have been measured and whose molecular weights are
thus known. The molecules of these compounds can contain
a small whole number of atoms of oxygen but never fractional
numbers. The percentage by weight of oxygen in these compounds
can be found by chemical analysis. This percentage of the molal
weight will be the number of grams of oxygen in a mole of the
compound, and this number must be either the atomic weight
of oxygen or some small even multiple thereof. In the above