An element with density 11 . 2 g cm - 3 forms a f . c . c . lattice with edge length of 4 × 10 - 8 cm . Calculate the atomic mass of the element (Given: N A = 6 . 022 × 10 23 mol - 1 ).

Given: Density of the element ( ρ ) = 11 . 2 g cm - 3 Z is the number of lattice points per unit cell. As it is given element forms a face-centred lattice, the effective number of atoms per unit cell is four. Edge length of the unit cell, a = 4 × 10 - 8 cm N A = 6 . 022 × 10 23 mol - 1 We know the density of unit cell, ρ = Z × M a 3 × N A M = ρ × a 3 × N A Z Substituting the values in the above formula, M = 11 . 2 × 4 × 10 - 8 3 × 6 . 022 × 10 23 4 = 107 . 9 g mol - 1 Hence, atomic mass of the element is 107.9 g m o l - 1 .

An element with density 11 . 2 g cm - 3 forms a f . c . c . lattice with edge length of 4 × 10 - 8 cm . Calculate the atomic mass of the element (Given: N A = 6 . 022 × 10 23 mol - 1 ).

Given: Density of the element ( ρ ) = 11 . 2 g cm - 3 Z is the number of lattice points per unit cell. As it is given element forms a face-centred lattice, the effective number of atoms per unit cell is four. Edge length of the unit cell, a = 4 × 10 - 8 cm N A = 6 . 022 × 10 23 mol - 1 We know the density of unit cell, ρ = Z × M a 3 × N A M = ρ × a 3 × N A Z Substituting the values in the above formula, M = 11 . 2 × 4 × 10 - 8 3 × 6 . 022 × 10 23 4 = 107 . 9 g mol - 1 Hence, atomic mass of the element is 107.9 g m o l - 1 .

An element with molar mass 27 g mol - 1 forms a cubic unit cell with edge length 4 . 05 × 10 - 8 cm . If its density is 2 . 7 g cm - 3 , what is the nature of the cubic unit cell ?

Number of atoms per unit cell, z = ρ × a 3 × N A M (equation 1 ) Here, ρ is the density of the cubic unit cell a is the edge length of the unit cell M is the molar mass of the element N A is the Avogadro's number Given, ρ = 2 . 7 g cm - 3 M = 27 g mol - 1 N A = 6 . 023 × 10 23 mol - 1 a = 4 . 05 × 10 - 8 cm Now, substituting these values in the expression ( 1 ) z = ( 2 . 7 g cm - 3 ) ( 4 . 05 × 10 - 8 cm ) 3 ( 6 . 023 × 10 23 mol - 1 ) 27 g mol - 1 = 4 So, the number of atoms in a unit cell, z = 4 Hence, it has face-centred cubic unit cell.

ZnO turns yellow on heating. Why?

In ZnO , Zn + 2 ions occupy the interstitial sites and electrons are trapped in the interstitial sites for neutralization. Crystals that show such type of defects are generally coloured. Zinc oxide is white in colour at room temperature. On heating, it loses oxygen and turns yellow.