And what does this mean? Well, the mass of #"Avogadro's Number"#, i.e. #6.022xx10^23# of INDIVIDUAL #MgCl_2# formula units have a mass of #95.21*g#. And thus the mole is the link between the micro world of atoms and molecules, that which we cannot observe but whose existence we cannot infer, with the macro world of litres, grams, and kilograms.........

And this is the principle of chemical equivalence. We divide the mass by the molar mass, and multiply the resultant #"molar"# quantity by the #"Avocado number"# we gets.......

#(17.2*cancel(g))/(95.21*cancelg*cancel(mol^-1))xx6.022xx10^23*cancel(mol^-1)=1.09xx10^23*"formula units"# (a number as required).

How many ATOMS are these in this quantity?