ollowing compounds is necessary to
prepare 50 l. of oxygen? (a) water; (b) mercuric oxide; (c)
potassium chlorate.

7. Reduce the following volumes to 0 deg., the pressure remaining constant:
(a) 150 cc. at 10 deg.; (b) 840 cc. at 273 deg..

8. A certain volume of gas is measured when the temperature is 20 deg.. At
what temperature will its volume be doubled?

9. Reduce the following volumes to standard conditions of pressure, the
temperature remaining constant: (a) 200 cc. at 740 mm.; (b) 500 l.
at 380 mm.

10. What is the weight of 1 l. of oxygen when the pressure is 750 mm.
and the temperature 0 deg.?

11. Reduce the following volumes to standard conditions of temperature
and pressure: (a) 340 cc. at 12 deg. and 753 mm; (b) 500 cc. at 15 deg. and
740 mm.

12. What weight of potassium chlorate is necessary to prepare 250 l. of
oxygen at 20 deg. and 750 mm.?

13. Assuming the cost of potassium chlorate and mercuric oxide to be
respectively $0.50 and $1.50 per kilogram, calculate the cost of
materials necessary for the preparation of 50 l. of oxygen from each of
the above compounds.

14. 100 g. of potassium chlorate and 25 g. of manganese dioxide were
heated in the preparation of oxygen. What products were left in the
flask, and how much of each was present?

CHAPTER III

HYDROGEN

~Historical.~ The element hydrogen was first clearly recognized as a
distinct substance by the English investigator Cavendish, who in 1766
obtained it in a pure state, and showed it to be different from the
other inflammable airs or gases which had long been known. Lavoisier
gave it the name hydrogen, signifying water former, since it had been
found to be a constituent of water.

~Occurrence.~ In the free state hydrogen is found in the atmosphere, but
only in traces. In the combined state it is widely distributed, being a
constituent of water as well as of all living organisms, and the
products derived from them, such as starch and sugar. About 10% of the
human body is hydrogen. Combined with carbon, it forms the substances
which constitute petroleum and natural gas.

It is an interesting fact that while hydrogen in the free state
occurs only in traces on the earth, it occurs in enormous
quantities in the gaseous matter surrounding the sun and
certain other stars.

~Preparation from water.~ Hydrogen can be prepared from water by several
methods, the most important of which are the following.