Composition of Hydrates
are ionic compounds (salts) that have a definite amount of water as part
of their structure.
This water is called water of hydration and is chemically combined
with the salt in a definite ratio.
The amount of water varies in different hydrates, but is
very specific for any given hydrate.
The formula for a hydrate is written as follows:
where the dot indicates that 2 molecules of water are loosely bonded to
one formula unit of the salt (BaCl
This formula, like all other formulas, indicates the
law of definite composition (all compounds have a definite proportion by mass).
When hydrates are heated, the water of hydration is released as water vapor.
remaining solid is known as the
The general reaction for heating a
Hydrate + Heat
Anhydrous Salt + Water
The percent of water in a hydrate can be found experimentally by accurately
determining the mass of the hydrate before heating it and then the mass of the anhydrous
salt after heating.
The difference in mass is due to the water lost.
The percentage of
water in the original hydrate can be calculated:
% Water =
(Mass Water/Mass of hydrate)
To remove the water from a hydrate and determine the percent by mass of
water in a hydrate.