Answer
Solubility rules are a set of empirical rules that chemists have inferred from observations on many ionic compounds. They are useful because they allow us to predict if a compound is soluble or insoluble.
Work Step by Step
These rules are guidelines for how to predict, off molecular formula, whether or not a compound will be soluble. Some entire groups on the periodic table, due to their electron arrangements in common, can determine whether a compound in which they are included is likely or unlikely to be soluble.