Question

Ice crystallises in a hexagonal lattice having volume of the unit cell as $122 \, \times \, 10^{- 24} \, cm^{3}$ . If its density is $0.90 \, g \, cm^{- 3}$ at a given temperature, then number of $H_{2}O$ molecules per unit cell is/are

• A. $1$
• B. $2$
• C. $3$
• D. $4$

Answer 1

Answer: (D)

$\frac{m a s s}{v o l u m e}= \, d$
Mass of $n$ molecules of $H_{2}O$ in $hexagonal \, packing=\frac{18 n}{N_{0}}=\frac{18 n}{6.02 \, \times \, 10^{23}}$
$= \, \frac{18 n}{6.02 \times 1 0^{23} \times 122 \times 1 0^{- 22}}= \, 0.90$
$n=4$