Number of atoms present in $224 \, dm^3$ of oxygen gas at $STP$ is

There are $6.02 \times 10^{22}$ molecules each of $N_2$,$O_2$,and $H_2$ which are mixed together at $760 \ mm$ and $273 \ K$. The mass of the mixture in grams is:

Which of the following has the largest number of atoms?

$6.02 \times 10^{20}$ molecules of urea are present in $100 \ mL$ of its solution. The concentration of urea solution is ......... $M$.

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.

$6.02 \times 10^{20}$ molecules of urea are present in $200 \ mL$ of its solution. The concentration of urea solution is .............. $M$