First, which type of bond must involve the interaction of more than one molecule? I would say it should be a hydrogen bond because it involves the sharing between two molecules. Is that correct?
No, it should be non-covalent (it can be H-bridge, salt bridge, but also van der Waals interaction etc.). Unless it’s not covalent bond (or intramolecular non-covalent bond), there are two molecules binding 😉
Next, how many covalent bonds are there in H3O+? And same question for PO43-? I think it would be four for the former, and 5 for the latter. Is this correct?
Where, the hell, did you get four covalent bonds in H3O+? Just conect the hydrogens to oxygen, how many bonds you have?
Phosphate is fairly more complicated. Three oxygens are bound with single bond (those, who care negative charge), and the last one is bound with single pi p-d bond (‘p-d’ are the orbitals involved in this bond; d in phosphorus and p in oxygen; this explains, why these bonds are only from third period down;))
How many H-bonds can 3 water molecules form (assuming a single H-bond per molecule pair)? What about for four water molecules?
From looking at a diagram in my textbook, I think the answer is two for both. Am I on the right track?
Why should four water molecules form only two H-bonds as do three molecules? Why not one more? And maybe, they could form even four hydrogen bonds, but I’m not sure, how much flexible they are