144 NON-METALLIC ELEMENTS IN BINARY COMPOUNDS
reaction will form a solid cake and the flask may be broken in
trying to remove it. It is important, however, to allow the con-
tents of the flask to cool nearly to room temperature before the
mixture of acid and acid sulphate are poured into water.
When the hydrochloric acid has cooled to room temperature
determine the volume and the specific gravity of the solution in
both absorption bottles. Use either a hydrometer or the apparatus
shown in Fig. 19. Consult the table on page 371 and determine
the percentage composition of the preparation. Calculate the
weight of hydrogen chloride obtained, the normality of the solu-
tion, and the percentage yield.
Practically all the hydrochloric acid will be found in the first
absorption bottle. Transfer this to a 250-cc. narrow-necked
glass-stoppered bottle.
QUESTIONS- How much of an excess of sulphuric acid was used in this
preparation? What is the advantage of using such an excess? - Why was air drawn through the apparatus after the reaction
was complete? - What conditions are necessary for the formation of Na 2 SO4
in the preparation of HC1? Why is NaHSO 4 called an acid salt?
PREPARATION 8
HYDROBROMIC ACID, HBr
The obvious method for preparing hydrobromic acid would be
to treat solid sodium bromide with concentrated sulphuric acid.
This is not a successful method, however, owing to the fact that
hydrogen bromide reacts to some extent with concentrated sul-
phuric acid according to the equation
H2SO4 + 2HBr -> 2H 2 O + SO 2 + Br 2
and the gas passing over to the absorption bottles would be con-
taminated with sulphur dioxide and bromine. To prepare pure
hydrobromic acid it is necessary to employ a different procedure.
The method we shall use starts with bromine and allows it to
react with red phosphorus and water. Phosphorus and bromine
combine very easily to form PBr3, but the phosphorus bromide
hydrolyzes completely and hydrogen bromide passes over to the
absorption flask.
PBr 3 + 3H 2 O -y P(OH) 3 + 3HBr