Air contains $23\% \, O_2$ and $77\% \, N_2$ by weight. What is the percentage of $O_2$ by volume?

$10 \ mL$ of $2 \ M$ $NaOH$ solution is added to $20 \ mL$ of $1 \ M$ $HCl$ solution kept in a beaker. Now,$10 \ mL$ of this mixture is poured into a volumetric flask of $100 \ mL$ containing $2 \ moles$ of $HCl$ and made the volume up to the mark with distilled water. The solution in this flask is $:$

Calculate the number of moles of $NaOH$ required to completely neutralize $100 \ g$ of $118\%$ oleum.

At $T(K)$,a gaseous mixture contains $H_2$ and $O_2$. The total pressure of the mixture is $2 \ bar$. The partial pressure of $H_2$ is $1.778 \ bar$. What is the weight $(w/w)$ percentage of $H_2$ in the mixture?

$0.05 \ cm$ thick coating of silver is deposited on a plate of $0.05 \ m^2$ area. The number of silver atoms deposited on the plate is . . . . . . $\times 10^{23}$. (Atomic mass of $Ag = 108$,density $d = 7.9 \ g \ cm^{-3}$)

The number of moles of solute present in the solutions of $I$,$II$ and $III$ is respectively:
$I$. $500 \ mL$ of $0.2 \ M \ NaOH$
$II$. $200 \ mL$ of $0.1 \ N \ H_2SO_4$
$III$. $6 \ g$ of urea in $1 \ kg$ of water