Question

A crystalline solid $AB$ adopts sodium chloride type structure with edge length of the unit cell as 745 pm and formula mass of 74.5 a.m.u. The density of the crystalline compound is:

• A. $2.16gc{m}^{-3}$
• B. $0.99gc{m}^{-3}$
• C. $1.88gc{m}^{-3}$
• D. $1.97gc{m}^{-3}$

Answer 1

Answer: (D)

$1.97gc{m}^{-3}$

Density $=\rho =\frac{Z\times M_A}{N_A\times a^3}$

$N_A=6.023\times {10}^{23}$

$Z_{NaCl}=4$ (FCC structure)

$M_{AB}=74.5$ $amu$

$a=7.45\mathring{A}$

$\rho =\frac{4\times 74.5}{6.023\times {10}^{23}\times (7.45{)}^3\times {10}^{-24}}=1.197$ $g/c{m}^3$