If NA is the Avogadro constant, how many valence electrons are present in 4.2 g N³⁻ ions?

4.2 g N³⁻ = 4.2/14 = 0.30 mol ions. Each N³⁻ ion has 8 valence electrons. Valence electrons = 0.30 × 8 = 2.4 mol electrons = 2.4NA electrons.