For this type of problem, we use the molar mass and Avogadros number in order to solve the question. 

Mass of compound -> Moles compound -> number of particles

this step is vice versa.

Let me show you an example.

50 grams of KCl

Solution:

50.0 grams KCl -> moles KCl -> number of particles

 `50 g KCl x ((1mol e KCl)/(74.55g KCl)) x ((6.022x10^23 particl es)/(1mol e KCl))`

= 4.04x10^23 particles of KCl

Now to work on from particles to the molar mass of compound, you just have to reciprocate the values. For example 4.00x10^23 atoms/particles of silver (Ag)

 `4.00x10^23 particl es Ag x ((1mol e Ag)/(6.022x10^23 particl es)) x ((107.87 g Ag)/(1mol e Ag))`

=71.65 grams Ag