Question

A raindrops has a mass of 0.50g. how many moles of water does a raindrop contain

Answer 1

So, here you have to look at the periodic table and see what the molar mass of each molecule composing the water compound $H_2 O$
its two hydrogen atom plus an oxygen molecule, making it about 18g per mol.

That said, looking at the mass, we have to ask ourselves, if a mol of water contains 18g of mass, how many moles do we have in 0.5g of water? 
We just do a cross multiplication:
$\frac{(0.5)}{18} =0.027 mol$