How many unit cells are present in a cube-shaped ideal crystal

How many unit cells are present in a cube-shaped ideal crystal of NaCl of mass 1.00 g? [Atomic masses: Na = 23, Cl = 35.5]

  1. 2.57 × 1021
  2. 1.28 × 1021
  3. 1.71 × 1021
  4. 5.14 × 1021

Answer

Mass (m) = density x volume = 1 g

Mol wt of NaCl = 58.5 g

Number of unit cell present in a cube shaped crystal of NaCl of mass 1 g = m x Na / (M x Z)

In NaCl each unit cell has 4 NaCl units.

Z = 4

The correct option is A.