the formula mass of MgCl - atomic mass of Mg + 2(atomic mass of Cl)
formula mass - 24.3 + (35.45x2) = 95.2
In 95.2 of MgCl₂ - there are 6.022 x 10²³ formula units of MgCl₂
Therefore in 15.0 g of MgCl2 - there are 6.022 x 10²³ /95.2 x 15.0 = 9.49 x 10²² formula units of MgCl₂
the number of formula units - 9.49 x 10²² formula units of MgCl₂