The pyknometric density of sodium chloride crystal is 2.165 × 10^3...

Molar volume from pyknometric density $=\frac{\mathrm{M}}{2.165 \times 10^3} \mathrm{~m}^3$ Molar volume of X-ray density $=\frac{\mathrm{M}}{2.178 \times 10^3} \mathrm{~m}^3$ Volume unoccupied $=\frac{M}{10^3}\left(\frac{1}{2.165}-\frac{1}{2.178}\right) \mathrm{m}^3$ Fraction occupied $$\begin{aligned} & =\frac{0.013 \times 10^{-3}}{2.165 \times 2.178} \times \frac{2.165}{\mathrm{M} \times 10^{-3}} \\ & =5.96 \times 10^{-3}\end{aligned}$$ Related Theory Pyknometric density is the closest approximation to true density calculated from the molecular weight and crystalline lattice of a substance.