how many formula units make up 22.2g of MgCl2?

The mole mass of MgCl2 is the sum of atomic masses of Mg + Cl + Cl = 24.305 + 35.453 + 35.453 = 95.211 g/mol
Using dimensional analysis, we can convert 22.2 g of MgCl2 to moles of MgCl2 to formula units of MgCl2. We shall need the following equalities to set up conversion factors:
1 mol MgCl2 = 95.211 g
1 mol MgCl2 = 6.02 × 10^23 formula units MgCl2
[(22.2 g MgCl2)/1][(1 mol MgCl2)/(95.211 g MgCl2)][(6.02 × 10^23 formula units MgCl2)] = 1.4036611 × 10^23 formula units MgCl2 or 1.40 × 10^23 formula units MgCl2 rounded to three significant figures
Answer: About 1.40 × 10^23 formula units of MgCl2 make up 22.2 g of MgCl2.

Molar mass of MgCl₂ = 24.31 + 35.45×2 = 95.21 g/mol
No. of moles of MgCl₂ = (22.2 g) / (95.21 g/mol) = 0.2332 mol

1 mole of MgCl₂ contains 6.022 × 10²³ formula units of MgCl₂.
No. of formula units of MgCl₂ = (0.2332 mol) × (6.022 × 10²³ /mol) = 1.404 × 10²³