How many atoms of niobium are present in $$2.43 \mathrm{~g}$$ if it forms bcc structure with density $$9 \mathrm{~g} \mathrm{~cm}^{-3}$$ and volume of unit cell $$2.7 \times 10^{-23} \mathrm{~cm}^3$$ ?

How many tetrahedral voids are present in 0.4 mole of a compound that forms hcp structure?

What is the number of octahedral and tetrahedral voids presents respectively in 0.25 mole of a substance having hcp structure?

What is the molar mass of a metal having density 8.57 g cm$$^{-3}$$ and edge length 3.3 $$\mathop A\limits^o $$ ? (packing efficiency = 68%)