24 QUANTITATIVE ASPECTS OF CHEMISTRY
combine to form zinc oxide the ratio is found to be 1.00 of oxygen
to 4.09 of zinc. No matter how much zinc or oxygen is used, the
amount of the one or the other in excess of this ratio will be left
over after combination has taken place.
Some substances do not combine with each other but each may
combine with a third. Under such conditions a ratio may be
calculated between the two substances that did not combine.
Fluorine will not combine directly with oxygen, but each of these
elements will combine with hydrogen. In hydrofluoric acid the
ratio by weight is 1.008 of hydrogen to 19.00 of fluorine. In
water the ratio by weight is 1.008 of hydrogen to 8.00 of oxygen.
The "combining ratio of oxygen to fluorine might be expected to be
8 :19 or 1: 2.375.
- DETERMINATION OF THE COMBINING RATIO OF ZINC
AND OXYGEN
Zinc and zinc oxide are both substances the weights of which
can be determined accurately, so that the quantity of oxygen which
is combined in the oxide can be found by difference. To convert
quantitatively a definite amount of zinc into the oxide by means
of direct combination with oxygen would be a difficult operation;
but the same result is accomplished indirectly by first treating
the metal with nitric acid to obtain the nitrate and then decom-
posing the zinc nitrate by heat, which leaves a residue of zinc
oxide.
Materials: zinc, chemically pure, about 20 mesh,
nitric acid, 6iV.
Apparatus: 4-inch porcelain dish.
5-inch watch glass. •
600-cc. beaker.
Bunsen burner,
triangle, nichrome wire,
iron ring and ring stand.
Copy the following form on the left-hand page of your'note
book for recording data :
Weight of evaporating dish + zinc grams
Weight of empty evaporating dish grams
Weight of zinc grams