Which of the following is the Loschmidt number?

The number of hydrogen atoms present in $5.4 \ g$ of urea is : (Given : Molar mass of urea: $60 \ g \ mol^{-1}$,$N_{A} = 6.022 \times 10^{23} \ particles \ mol^{-1}$)

The number of moles of electrons in $1 \ kg$ of electrons is .......

The number of oxygen atoms in $4.4 \ g$ of $CO_2$ is approximately ........ $\times 10^{23}$.

How many atoms are contained in $5.6 \ L$ of oxygen gas at $STP$?

Calculate the number of valence electrons in $4.2 \ g$ of $N_3^-$ ions in terms of $N_A$ ($N_A$ = Avogadro constant). (in $N_A$)