30 QUANTITATIVE ASPECTS OF CHEMISTRYhydrogen is evolved from the metal. The cone of the funnel
should always be kept well filled with liquid, and care must be
taken that no bubble of air is sucked into the stem. Collect the
hydrogen in the flask, and when the gas has forced the level of
the liquid nearly into the neck begin to wash the acid out of the
apparatus by pouring water into the funnel. Have the acid com-
pletely removed, and thus the evolution of hydrogen stopped by
the time the water level stands at about the middle of the neck.
Equalize the level inside and outside the neck of the flask, and
while in this position mark the level by means of a rubber band.
Record temperature and barometric pressure. Determine on the
platform scales the weight of water required to fill the volume
occupied by the hydrogen. Dry the zinc carefully on a piece of
filter paper, and weigh it accurately on the sensitive balances.
Calculation. (1) Calculate the volume of hydrogen under stand-
ard conditions equivalent to 1 gram of zinc. (2) From the com-
bining ratio of zinc and oxygen (Experiment 1), find the volume
of hydrogen equivalent to 1 gram of oxygen. (3) From the weight
of 1 liter of oxygen (Experiment 2), find the volume of hydrogen
equivalent to 1 liter of oxygen. Note that the calculation of the
ratios (2) and (3) is based on an assumption, namely, that the
quantity of hydrogen displaced by a given weight of zinc would
combine with the same weight of oxygen that would combine with
the given weight of zinc.
- THE COMBINING RATIO OF HYDROGEN AND
OXYGEN IN WATER
It is only with great care and refined apparatus that volumes of
gases can be successfully weighed. It not being feasible to weigh
hydrogen and oxygen as such in this experiment, the oxygen will
be obtained from solid copper oxide, the loss of weight of which can
be determined. If an excess of dry hydrogen is led over the copper
oxide, the excess that does not combine with the oxygen to form
water will pass on unchanged. By collecting all the water in
some material which absorbs it, its weight may be found. The
amount of the hydrogen combined is then given by the differ-
ence between the weight of the water and that of the oxygen.
Hydrogen gas generated from zinc and hydrochloric acid is
passed through a tube containing calcium chloride to remove any
water vapor, then over heated copper oxide, with the oxygen of