An ideal $NaCl$ crystal has density $2.165 g c m^{- 3}$ . In a real crystal $0.50%$ of the formula units have a Schottky pair missing, with no change in cell volume. Find the density of the real crystal in $g c m^{- 3}$ (two decimals).

Numerical answer type, answer in g/cm^3, tolerance 0.01.

Answer: 2.15 g/cm^3

Each missing pair removes one formula unit's mass, and the volume is unchanged. Density

= 2.165 \times (1 - 0.005) = 2.1542 g c m^{- 3} \approx 2.15