Question

Calculate total number of valence electrons in 2.8 g of N$^{3-}$ ion

• A. 2.6 N$_A$
• B. 1.6 N$_A$
• C. 7.8 N$_A$
• D. 0.8 N$_A$

Answer 1

Answer: (B)

1.6 N$_A$

Answer 2

For the nitride ion $N^{3-}$:

• Atomic mass of N: 14 g/mol

• Moles of $N^{3-}$ in 2.8 g:

  $$\text{moles} = \frac{2.8}{14} = 0.2$$

• Valence electrons in one $N^{3-}$ ion:

  Nitrogen has 5 valence electrons, and the $3-$ charge adds 3 extra electrons, so total = $5 + 3 = 8$ electrons.

• Total valence electrons:

  $$\text{Total electrons} = 0.2 \times 8 \times N_A = 1.6\,N_A$$