WEIGHT OF A LITER OF OXYGEN 25Weight of evaporating dish + zinc oxide
after first heating grams
second heating grams
third heating grams
final weight grams
Weight of evaporating dish + zinc grams
Weight of oxygen grams
Procedure: Weigh accurately a 4-inch porcelain evaporating
dish. Place in it about 1 gram of pure zinc, and again weigh ac-
curately. Add 5 cc. of water, and cover the dish with a 5-inch
watch glass. Add 5 cc. of dilute nitric acid (6iV). Note the
nature of the reaction which takes place and the color of the gas
which is evolved. If the reaction stops completely before the
zinc has entirely dissolved add 3 cc. more of dilute acid. When
solution is complete remove the watch glass and, with a jet of
water from the wash bottle, rinse the drops of liquid clinging from
the under side into the dish. Place the solution on a water bath
(beaker of boiling water), and leave it to evaporate (with the
watch glass removed) as much as possible. When only a small
amount of sirupy liquid (melted zinc nitrate) is left, take the dish
to the hood, support it on a wire triangle, and heat carefully with
a very small flame, holding the burner in the hand. If the liquid
starts to boil, remove the flame at once, because every tiny drop
that spatters out of the dish means a loss of material. Heat until
the mass thickens and red fumes escape, and finally, after the mass
becomes perfectly dry, heat quite strongly for a few minutes.
Cool and weigh. Again heat quite strongly and weigh. The
weight ought not to have decreased, but if it has, the heating must
be continued until the weight becomes constant.
Calculation. From the weight of the zinc and the zinc oxide
find the combining ratio of oxygen and zinc.- WEIGHT OF A LITER OF OXYGEN
Oxygen is generated by heating potassium chlorate, and its
weight is determined by the loss in weight of that material.
Since the gas is collected over water, under the conditions of
temperature and pressure prevailing in the laboratory, the volume
must be corrected to standard conditions by means of the formula
on page 44.