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A cube-shaped perfect crystal of NaCl has a mass of 1.00 g. How many unit cells does it contain? [Atomic masses: Na = 23, Cl = 35.5]

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

Correct answer: 2.57 × 10²¹ unit cells

Solution

Formula units = (1/58.5)*6.022e23 = 1.03e22; each NaCl unit cell contains 4, so cells = 1.03e22/4 = 2.57e21 (option 3), not 1.71e21.

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