DETERMINATION OF MOLECULAR WEIGHT 33gen a little, being careful in adding acid to pour it in a thin stream
down the side of the tube so as not to drag with it bubbles of air,
as this would introduce oxygen, which would be burned to water
in the combustion tube. Let the hydrogen generate 3 minutes
longer to sweep any last traces of oxygen from the apparatus,
and then begin heating the copper oxide rather gently, using the
wing-top burner. Before the experiment is ended any con-
densed water must be completely driven over into the calcium
chloride tube by playing the flame gently over the parts of the
tube where moisture is seen. When the tube has cooled to room
temperature, disconnect the hydrogen generator from the drying
tube and blow slowly enough air (from the lungs) through the
apparatus to displace all the hydrogen. Weigh the boat to find
the loss, and the calcium chloride tube to find the increase in weight.
Calculation. Calculate the combining ratio of hydrogen and
oxygen, that is, how many parts by weight of oxygen combine
with 1 part of hydrogen.
From this ratio and the volume ratio calculated in Experiment
3, find the weight of 1 liter of hydrogen. This involves the
assumption that the quantity of hydrogen displaced by that
weight of zinc which combines with a given amount of oxygen is
the same as would combine directly with that amount of oxygen.
By comparing the result obtained with the known weight of a
liter of hydrogen (= 0.090 gram) decide whether this assumption is
correct.
- DETERMINATION OF THE APPROXIMATE MOLECULAR WEIGHT
OF A VOLATILE LIQUID BY DUMAS' METHOD
According to Avogadro's principle, the volume occupied by the
gram molecular weights of all gases, under standard conditions,
is the same. Determinations made with a large number of gases
have shown that this volume is 22.4 liters. For example, 2.016
grams of hydrogen, or 32 grams of oxygen, or 71 grams of chlorine
occupy a volume of 22.4 liters at 0° and 760 mm. pressure. If a
new gas is discovered, its molecular weight can be determined by
finding the weight that occupies a volume of 22.4 liters under
standard conditions.
This method can be applied also to substances which are liquid
or solid at 0° and 760 mm. pressure, provided they can be vapor-