The total number of valence electrons in $4.2 \ g$ of $N_3^-$ ion is ............. $N_A$ ($N_A$ is the Avogadro's number).

How many moles of electrons weigh $1 \ kg$?

The number of neutrons in $5 \ g$ of $D_2O$ ($D$ is $_1^2H$) are ................ $N_A$.

How many $g$ of $H_2SO_4$ are present in $0.25 \, \text{mol}$ of $H_2SO_4$?

$A$ thin layer of cobalt is deposited on a plate with a total area of $0.5 \ m^2$ and a thickness of $0.005 \ cm$. How many cobalt atoms are deposited on the plate? (Density of cobalt $= 8.9 \ g \ cm^{-3}$,Atomic mass of $Co = 59 \ g \ mol^{-1}$)

If it takes $1$ second to count $4$ grains of wheat,then the time required to count one mole of wheat grains is ...... years.