Question

How many moles of MgS2O3 are in 205 g of the compound?

Answer 1

$Mol = \frac{mass}{molar mass}$

Mass of the compound = 205 g
Molar Mass of compound = ((24) + (2 * 32) + (3 * 16))
                                         =  136 g / mol

∴ # mols in 205g =  $\frac{205 g}{136 g / mol}$
                           = 1.507 mol 

Answer 2

Since a mol of MgS2O3 weights 136gr (the same weight as the atomic weight for that compound, but measured in gr), you just have to divide the mass you have (205) by that one (136), resulting in 1'514 moles.