Question: how many molecules are there in ammonia?
Answer: Ammonia, formed of 3 hydrogen atoms and a nitrogen atom, has an atomic weight of 17g/mol. (Actually 17.031g/mol if we're being picky.) This means that 17g of ammonia is one mole of ammonia. The number of molecules in a single mole of any compound is given by Avoadro's constant, which is 6.023x10^23.