Copper(I) iodide crystallizes with the zinc blende structure. If the edge length of the unit cell is 605 pm, what is the density (in g/cm³) of crystalline CuI?

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**You are asked** to calculate the density of a solid.

**You are given** the identity and crystal structure of the solid and the edge length of the unit cell.

Density is calculated from the mass and the volume of the unit cell.

**Step 1.** Calculate the mass of the unit cell using the fact that there are four formula units per unit cell in the ZnS structure.

= 1.27 \times 10^{-21} g/unit cell

**Step 2.** Calculate the volume of the unit cell (in cm³) from the edge length.

edge length = 605 pm

605 pm \times\frac{10^{2}\text{ cm}}{10^{12 }\text{ pm}} =6.05\times 10^{-8} cm

volume = a³ = \left(6.05\times 10^{-8}\text{ cm}\right)^{3}=2.21\times 10^{-22} cm³

**Step 3.** Calculate density from the mass and volume of the unit cell.

density = \frac{\text{mass}}{\text{volume}}=\frac{1.27\times 10^{-21}\text{ g}}{2.21\times 10^{-22}\text{ cm}^{3}} = 5.71 g/cm³