How many atoms are in 4 moles of MgCl2?

1 mole of any substance contains 6.02 x 1023 constituent particles, correct to three significant figures (avogadro constant).

Number of molecules of 4 moles of MgCl2 = 4 x 6.02 x 1023

In every molecule of MgCl2 there are 3 atoms.

Thus, number of atoms in 4 moles of MgCl2 = 3 x 4 x 6.02 x 1023 = 7.22 x 1024