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

Which of the following has the smallest number of molecules?

The mass of $0.1 \ mol$ of methane $(CH_4)$ is .......... $g$.

The number of moles of $SO_2Cl_2$ in $13.5 \ g$ is

The number of sodium atoms in $2 \ mol$ of sodium ferrocyanide $Na_4[Fe(CN)_6]$ is .......... $\times 10^{23}$. (Given $N_A = 6.022 \times 10^{23} \ mol^{-1}$)