Which of the following reactions exhibits good atom economy according to the principles of green chemistry?

The molarity of $HNO_{3}$ in a sample which has density $1.4 \; g/mL$ and mass percentage of $63\%$ is . . . . . . $M$.
(Molecular Weight of $HNO_{3} = 63$)

$50 \ mL$ of $0.5 \ M$ oxalic acid is needed to neutralize $25 \ mL$ of sodium hydroxide solution. The amount in grams of $NaOH$ in $50 \ mL$ of the given sodium hydroxide solution is:

The mass percentage composition of dry air at sea level is approximately $N_2: 63 \%$, $O_2: 16 \%$, $Kr: 21 \%$. If the total pressure is $p \ atm$, calculate the partial pressure of each component respectively. (Molecular weights: $N_2 = 28, O_2 = 32, Kr = 84$)

At $300 \ K$ and $1 \ atm$,$15 \ mL$ of a gaseous hydrocarbon requires $375 \ mL$ of air containing $20 \% \ O_2$ by volume for complete combustion. After combustion,the gases occupy $330 \ mL$. Assuming that the water formed is in liquid form and the volumes were measured at the same temperature and pressure,the formula of the hydrocarbon is:

Calculate the approximate mass (in $g$) of $H_2S$ required for the following reaction when $15 \ L$ of oxygen at $STP$ reacts completely.
$2 H_2S_{(g)} + 3 O_{2(g)} \longrightarrow 2 SO_{2(g)} + 2 H_2O_{(g)}$
(molar mass of $H_2S = 34 \ g \ mol^{-1}$)