Question

A crystalline substance has density of 2 g/cc with fcc crystal lattice.Its edge length is 700 pm. The molecular mass of the crystalline substance in g/mol is :

• A. 51.2
• B. 206.6
• C. 103.2
• D. 74.5

Answer 1

The correct option is C 103.2

We have,

$a=700pm$ $=700\times {10}^{-10}cm$

for fcc, $z=4$

$density=\frac{ZM}{N_A\times a^3}$

$2=\frac{4M}{6.022\times {10}^{23}\times {\left(700\times {10}^{-10}\right)}^3}$

$6.022\times {10}^{23}\times {\left(700\times {10}^{-8}\right)}^3=2M$

$M=\frac{6.022\times {10}^{23}\times 343\times {10}^{-1}}{2}=103.2gmo{l}^{-1}$

Therefore, the option is C.