The formula of magnesium chloride is #"MgCl"_2"#.
The molar mass of #"MgCl"_2"# is #"(24.30 + 2 × 35.45) g/mol" = "95.20 g/mol"#.
∴ #"Moles of MgCl"_2 = 39.8 color(red)(cancel(color(black)("g MgCl"_2))) × ("1 mol MgCl"_2)/(95.20 color(red)(cancel(color(black)("g MgCl"_2)))) = "0.4181 mol MgCl"_2#
There are #6.022 × 10^23# formula units in 1 mol, so
#"Formula units" = 0.4181 color(red)(cancel(color(black)("mol MgCl"_2))) × (6.022 × 10^23color(white)(l) "formula units MgCl"_2)/(1 color(red)(cancel(color(black)("mol MgCl"_2)))) = 2.52 × 10^23color(white)(l) "formula units MgCl"_2#
