what does the dot in CaSO4•2(H2O) mean ??

Answer 1

Good question. It's been called "the dot of ignorance" in the past, and it still kinda is. What it means on the most practical level is that if you have solid CaSO4•2(H2O), then stoichiometrically you have two molecules of water for each calcium and sulfate in the solid. Since that affects the molecular weight, if you want to make solutions, etc. you need to account for them. And if someone just wrote the chemicals as CaSH8O8, you'd not have any clue what it is, so separating the waters out explains better the nature of the chemical.

At a more philosophical level, what it means is that "we can determine there are two molecules of water in the solid for every molecule of CaSO4, but we don't understand why or how. So we'll just put that dot there to separate the stuff we care about (CaSO4 in this case) from the stuff we have to deal with (the two water molecules and how they affect the molecular weight, etc.)."

That notation predates the modern understanding of inorganic chemistry. Calcium, for example, can form six (or eight) coordinate bonds; it has certain stable geometries, and will form certain types of compounds but not others. Using modern understanding of inorganic chemistry and crystallography all of these behaviors can be explained, but since that notation predates this understanding it basically means "there are a bunch of water molecules here and we don't why." Hence, the dot of ignorance.

(As an aside, that notation is of course now used by modern inorganic chemists and crystallographers, and in sufficiently high levels, although not at the freshman chemistry-type level you're probably talking about, we still don't understand this fully. Someimes crystals will have certain solvent molecules in them that are almost illogical. It just happens. So even today there is an element of the dot of ignorance in it.)

Answer 2

CaSO4*2 H2O this is gypsum
this shows you that the CaSO4 compound is hidrated with 2 molecules of water.