Question

Selenium ingestion of $90$ micrograms per day causes loss of hair. How many selenium atoms are there in this sample? (Atomic weight of $Se=78.96$)

• A. $6.8\times {10}^{23}$ atoms
• B. $8.8\times {10}^{17}$ atoms
• C. $8.8\times {10}^{22}$ atoms
• D. $6.9\times {10}^{17}$ atoms

Answer 1

The correct option is D $6.9\times {10}^{17}$ atoms

Weight of selenium $=90\mu$ g $=90\times {10}^{-6}$ g.

Atomic weight of Se $=$ 78.96 g/mol.

The number of moles of selenium $=\frac{90\times {10}^{-6}}{78.96}=1.14\times {10}^{-6}$ mol.

Avogadro's number $=6.023\times {10}^{23}$ atoms/mol.
The number of atoms of selenium $=1.14\times {10}^{-6}\times 6.023\times {10}^{23}=6.9\times {10}^{17}$ atoms.