For example — "high purity chloride salt of Zinc $(\ce{ZnCl2.2H2O})$" or "various concentrations of $\ce{FeCl2.4H2O}$". What does the number before $\ce{H2O}$ mean?
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2 Answers
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The salt's crystal lattice's repeating unit is constituted of n molecules of salt and m molecules of water. Such salts are called hydrates. Wikipedia even has a nice picture of a hydrated vs non hydrated salt.
You can have anhydrous ferrous chloride, as well as ferrous chloride tetrahydrate. The number indicates how many water molecules are present in the crystal lattice's unit cell.
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This has just crossed my mind: be careful when preparing solutions of metal complexes to make sure you know what compound you're working with. One gram of anhydrous salt contains more equivalents than a gram of hydrated salt. This information should be obvious on the bottle. If not, ask a lab tech if they can identify the hydrated/anhydrous salt by memory.
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$\begingroup$ Note that the older notation for coordination compounds used a dot as well, before coord compounds were properly understood. $\endgroup$ Commented May 14, 2012 at 19:26
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You should look up water of crystallization.
These H2O molecules that are written after that salt are actually the water of crystallization, i.e., the no. Of water molecules that attach to one formula unit of a salt.
You may know about the fact that hydrated CuSO4 salt (blue vitriol) is blue in color. This salt has these same water of crystallization. Its formula is CuSO4.5H2O. But upon heating, it loses water of crystallization due to evaporation and simply reverts to its anyhydrous state CuSO4.
