The number of moles of oxygen in $1 \ L$ of air containing $21 \%$ oxygen by volume,at standard temperature and pressure $(STP)$,is (in $mol$)

Which of the following has the largest number of atoms?

Among the following,choose the ones with an equal number of atoms.
$A$. $212 \ g$ of $Na_2CO_3(s)$ [molar mass $= 106 \ g/mol$]
$B$. $248 \ g$ of $Na_2O(s)$ [molar mass $= 62 \ g/mol$]
$C$. $240 \ g$ of $NaOH(s)$ [molar mass $= 40 \ g/mol$]
$D$. $12 \ g$ of $H_2(g)$ [molar mass $= 2 \ g/mol$]
$E$. $220 \ g$ of $CO_2(g)$ [molar mass $= 44 \ g/mol$]
Choose the correct answer from the options given below.

Calculate the mass of oxygen present in $0.1 \ mol$ of $Na_2CO_3 \cdot 10H_2O$. (in $g$)

If $10^{21}$ molecules are removed from $200 \, mg$ of $CO_2$,then the number of moles of $CO_2$ left is .....

One molecule of water has a volume of ........ $\times 10^{-23} \text{ cm}^3$? (Density of water = $1 \text{ g cm}^{-3}$)