Question

$KF$ has $NaCl$ structure. What is the distance (in $A^o$) between $K^{+}$ and $F^{-}$ in $KF$, if density of $KF$ is $2.48$ $gm/c{m}^3$? $[K=39,F=19]$

• A. $2.86$
• B. $2.68$
• C. $3.16$
• D. $3.68$

Answer 1

Answer: (B)

$2.68$

Effective number of KF molecules is 4.

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

$⟹2.48=\frac{4\times 58}{6.023\times {10}^{23}\times a^3}$

$⟹a^3=155.3\times {10}^{-24}$

$⟹a=5.36\times {10}^{-8}$ $cm=5.36\mathring{A}$

In $NaCl$ type stucture distance between cation and anion $=\frac{a}{2}$

$⟹$ Distance between cation and anion $=\frac{a}{2}=2.68\mathring{A}$