Question

Platinum crystallises in a face centered cube crystal with a unit cell length of 3.9231 Å. The density and atomic radius of platinum are respectively. [Atomic mass of Pt = 195]

• A. 45.25 g. cm–3, 2.516 Å
• B. 21.86 g. cm–3, 1.387 Å
• C. 29.46 g. cm–3, 1.48 Å
• D. None of these

Answer 1

Answer: (B)

21.86 g. cm–3, 1.387 Å

Answer 2

Density = $\frac{Z \times M}{N_{A} \times a^{3}} = \frac{4 \times 195}{6.02 \times 10^{23} \times (3.9231 \times 10^{- 8})^{3}}$

= 21.86 g/cm3

for fcc lattice, 4r = $a\sqrt{2}$

so, r = $\frac{a\sqrt{2}}{4}$=$\frac{3.9231\sqrt{2}}{4}$Å = 1.387 Å.