Thanks, nice set of notes, but I don't see that the notes entail any information in regard to determine which compound, among a set of compounds is the _most_ or the _least_ soluble? How would that be determined?
There isn't a simple rule, chemistry in cases like this reveals its empirical nature: you have to know some results.
Anyway, with time, you will learn that:
1. K
3PO
4 is soluble, because there are just a few insoluble compounds of potassium and this one is not in the list. Or: phosphates of alcaline metals are soluble (apart from Li).
2. Fe(NO
3)
2 (but are you sure is not Fe(NO
3)
3 instead?) is certainly soluble because all nitrates are.
3. Na
2CO
3 is soluble because, apart from lithium, carbonates of alcaline metals are very soluble.
4. We are left with CuCl
2 and Hg
2Cl
2. Often, Cu
2+ salts are soluble and Hg
22+ are insoluble, but it's not a rule, so you have to know which are and which aren't.