An ionic compound crystallizes in a rock salt type structure. The...

For a rock salt (FCC) structure, the edge length $a$ is related to the sum of the cationic and anionic radii by:
$a = 2(r_+ + r_-)$

Given that $(r_+ + r_-) = 250 \text{ pm}$:
$a = 2 \times 250 \text{ pm} = 500 \text{ pm} = 5 \times 10^{-8} \text{ cm}$

The number of formula units per unit cell for a rock salt structure is $Z = 4$.

The density is given by:
$d = \dfrac{Z \times M}{N_A \times a^3}$

Rearranging for molar mass $M$:
$M = \dfrac{d \times N_A \times a^3}{Z}$

Substituting the given values:
$M = \dfrac{3.2 \times 6.0 \times 10^{23} \times (5 \times 10^{-8})^3}{4}$

$M = \dfrac{3.2 \times 6.0 \times 10^{23} \times 125 \times 10^{-24}}{4}$

$M = \dfrac{3.2 \times 6.0 \times 12.5}{4}$

$M = 0.8 \times 75 = 60 \text{ g mol}^{-1}$

The molar mass of the ionic compound is $60$.

Answer: $60$