Ncert Exercises & Solutions

1.12 A cubic solid is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body-centre. What is the formula of the compound? What are the coordination numbers of P and Q?

It is given that the edge length, a = 4.077 × 10⁻⁸ cm
Density, d = 10.5 g cm⁻³
As the lattice is fcc type, the number of atoms per unit cell, z = 4
We also know that, NA = 6.022 × 10²³ mol⁻¹

Using the relation:

d = \frac{zM}{a^{3} N_{A}}

\Rightarrow M = \frac{d a^{3} N_{A}}{z}

= \frac{10.5 g {cm}^{- 3} \times {(4.077 \times 10^{- 8} cm)}^{3} \times 6.022 \times 10^{23} {mol}^{- 1}}{4}

= 107.13 g {mol}^{- 1}

Therefore, atomic mass of silver = 107.13 u

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