A cubic ionic solid of formula mass 58.5 g mol^-1 has edge length 564 pm and density 2.16 g cm^-3. Using N_A = 6.022x10^23 mol^-1, the number of formula units per unit cell is

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A cubic ionic solid of formula mass $58.5 g mo l^{- 1}$ has edge length $564 pm$ and density $2.16 g c m^{- 3}$ . Using $N_{A} = 6.022 \times 1 0^{23} mo l^{- 1}$ , the number of formula units per unit cell is

Z = \frac{ρ N _{A} a ^{3}}{M} = \frac{2.16 \times 6.022 \times 1 0 ^{23} \times ( 5.64 \times 1 0 ^{- 8} ) ^{3}}{58.5} = 3.99 \approx 4