e density of sulphuric acid is 1.84. Express the acid
required in (b) in cubic centimeters.

9. What weight of potassium chlorate is necessary to furnish sufficient
oxygen to fill four 200 cc. bottles in your laboratory (the gas to be
collected over water)?

CHAPTER VII

NITROGEN AND THE RARE ELEMENTS: ARGON, HELIUM, NEON, KRYPTON, XENON

~Historical.~ Nitrogen was discovered by the English chemist Rutherford in
1772. A little later Scheele showed it to be a constituent of air, and
Lavoisier gave it the name _azote_, signifying that it would not support
life. The name _nitrogen_ was afterwards given it because of its
presence in saltpeter or niter. The term azote and symbol Az are still
retained by the French chemists.

~Occurrence.~ Air is composed principally of oxygen and nitrogen in the
free state, about 78 parts by volume out of every 100 parts being
nitrogen. Nitrogen also occurs in nature in the form of potassium
nitrate (KNO_{3})--commonly called saltpeter or niter--as well as in
sodium nitrate (NaNO_{3}). Nitrogen is also an essential constituent of
all living organisms; for example, the human body contains about 2.4% of
nitrogen.

~Preparation from air.~ Nitrogen can be prepared from air by the action of
some substance which will combine with the oxygen, leaving the nitrogen
free. Such a substance must be chosen, however, as will combine with the
oxygen to form a product which is not a gas, and which can be readily
separated from the nitrogen. The substances most commonly used for this
purpose are phosphorus and copper.

1. _By the action of phosphorus._ The method used for the preparation of
nitrogen by the action of phosphorus is as follows:

The phosphorus is placed in a little porcelain dish, supported on a cork
and floated on water (Fig. 26). It is then ignited by contact with a hot
wire, and immediately a bell jar or bottle is brought over it so as to
confine a portion of the air. The phosphorus combines with the oxygen to
form an oxide of phosphorus, known as phosphorus pentoxide. This is a
white solid which floats about in the bell jar, but in a short time it
is all absorbed by the water, leaving the nitrogen. The withdrawal of
the oxygen is indicated by the rising of the water in the bell jar.

[Illustration: Fig. 26]

2. _By the action of copper._ The oxygen present in the air may also be
removed by passing air slowly through a heated tube containing copper.
The copper combines wi