See also Entering Formulas.
Dashes are used to add hydrates to a compound. For example, FeCl3-6H2O means ferric chloride with 6 water molecules attached. As shown, an integer can follow the dash, though it's not required (in which case 1 is assumed to follow the dash). In fact, the dash can be used many times to add multiple hydrates onto a compound (if you have the desire). Also, other molecules besides hydrates can be added. For example (fictional compound) C6H6-H2O-2ZnHgMg-U would be benzene + water + 2*(zinc+mercury+magnesium) + uranium for a total molecular weight of 914.73. Decimal numbers can also be entered after a dash to add fractional parts of hydrates or other compounds, for example FeCl3-2.5H2O
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