1-18. WATERS OF HYDRATION
Many times when a substance crystallizes into a solid, molecules of water are
included in the crystal. These molecules of water combine with the substance in a fixed
ratio, similar to the fixed ratios between the atoms in a molecule. Whenever weighing or
doing calculations based on compounds that have waters of hydration, the amount of
water in the crystals must be taken into consideration.
a. When writing formulas for these compounds, the waters of hydration are
shown by placing a dot (or dash) after the formula for the compound, followed by the
formula for water with a coefficient to indicate the number of waters of hydration. For
example, cupric sulfate forms crystals that contain five molecules of water for each
molecule of cupric sulfate--its formula is written CuSO4.5H2O.
b. Compounds that contain waters of hydration are called hydrates. (If all the
water has been removed by drying, they are called anhydrous.) When writing the
names for these compounds, the number of waters of hydration is indicated by using
number prefixes. Thus, the name for CuSO4.5H2O is cupric sulfate pentahydrate.
Another number prefix seen occasionally in the names of hydrates is hemi-, which
means one-half (1/2).
(1)
Questions.
(a) AlCl3.6H2O is called ____________________________________ .
(b)
Mg3(PO4)2.5H2O is called ________________________________ .
(c)
Na2HPO4.7H2O is called ______________________________ .
(d) FeSO4.7H2O is called ___________________________________ .
(e) Na2CO3.1OH2O is called ________________________________ .
(f)
CaSO4.1/2H2O is called _________________________________ .
(2)
Answers.
(a) Aluminum chloride hexahydrate.
(b)
Magnesium phosphate pentahydrate.
(c)
Disodium hydrogen phosphate heptahydrate.
MD0803
1-31
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