Question

An element with density 2.8 g cm⁻³ forms fcc unit cell having edge length 4 × 10⁻⁸ cm. Calculate molar mass of the element.

Options

• 22.0 g mol⁻¹

• 27.0 g mol⁻¹

• 33.0 g mol⁻¹

• 36.0 g mol⁻¹

Answer 1

27.0 g mol⁻¹

Explanation:

ρ = 2.8 g cm⁻³

a = 4 × 10⁻⁸ cm

n = 4 (for fcc)

M = ?

ρ = `("n" xx "M")/("a"^3 xx "N"_"A")`

∴ M = `(ρ xx "a"^3 xx "N"_"A")/"n"`

∴ M = `(2.8 xx (4 xx 10^-8)^3 xx 6.022 xx 10^23)/4`

= 26.97 ≈ 27 g mol⁻¹