how do you determine what kind of compound is formed after a reaction?

Answer 1

You look at what's left over. When you are doing your reaction in aqueous solution, the ionic salts dissociate and then bounce about as ions.

If certain ions combine and precipitate, then they do not continue to mix in solution and precipitate out. Then the remaining ions form your new solution.

In other cases, your ions might liberate gas

eg. HCl + NaHCO3 -> CO2(g) + H2O(l) + Na+ + Cl-

which means the CO2 bubbles up, the H2O is just more water and you're left with Na+ and Cl- ions in aqueous solution

otherwise you just get a mix

Answer 2

That's extremely vague. For most reactions, you know what the product SHOULD be. The problem comes along where molecules do something that you didn't intend. That's where the fun comes, identifying the unknown.

1) depends on organic (NMR, MS, IR, MP) vs. inorganic (IR, MP, + several others).

For organic compounds, NMR (proton, carbon, NOESY, COSY, ROSY, etc) are used to determine structure. Mass Spec (MS) is used to determine molecular weight. IR and MP are old school and only used sparingly in industry. If you have chiral molecules, you use a polarimeter to determine rotation.

2) known vs. unknown, if the compound is known, you compare your product to several candidate's physical/spectral properties reported values. If a compound is a white crystalline solid and you have a yellow amorphous liquid, either a) not the same compound or b) your sample needs to be purified.