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# If $N_A$ is Avogadro's number then number of valence electrons in $4.2\;g$ of nitride ions $(N^{3-})$ is

$(a)\;4.2\;N_A \\ (b)\;2.4\;N_A \\(c)\;1.6\;N_A \\(d)\;3.2\;N_A$

Moles of nitride ion $=\large\frac{4.2}{14}$$=0.3\;mol$
$\qquad= 0.3 \times N_A$ molecules
$\therefore$ Total valence electrons in $N^{3-}$ ion $=5+3=8$
$\therefore$ No. of valence electrons $= 8 \times 0.3 N_A$
$\qquad= 2.4 \;N_A$