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# A unit cell of sodium chloride has four formula units.The edge length of the unit cell is 0.564nm.What is the density of sodium chloride?

$\begin{array}{1 1}2.166gcm^{-3}\\3.166gcm^{-3}\\4.166gcm^{-3}\\1.166gcm^{-3}\end{array}$

Answer : $2.166gcm^{-3}$
Using the formula $\rho=\large\frac{N}{a^3}\frac{M}{N_A}$
$\rho=\large\frac{4}{(0.564\times 10^{-7}cm)^3}\big(\frac{58.5gmol^{-1}}{6.023\times 10^{23}mol^{-1}}\big)$$=2.166gcm^{-3}$