The mass of water produced when $4 \ g$ of hydrogen reacts with $4 \ g$ of oxygen is .......... $g$.

$250 \ mL$ of $0.5 \ M \ NaOH$ was added to $500 \ mL$ of $1 \ M \ HCl$. The number of unreacted $HCl$ molecules in the solution after complete reaction is $...... \times 10^{21}$. (Nearest integer) $(N_{A} = 6.022 \times 10^{23})$

$A$ reaction gas mixture contains $50 \%$,$30 \%$,and $20 \%$ of $A$,$B$,and $C$ by volume,respectively. The mixture undergoes the following reactions:
$1. A + 2B \rightarrow P_1$
$2. 4P_1 + 3C \rightarrow P_2$
$P_1$ and $P_2$ are two products of the reactions. Choose the correct answer if the reaction completes.

$10 \, mL$ of $10 \, M$ $H_2SO_4$ is mixed with $100 \, mL$ of $1 \, M$ $NaOH$ solution. The resultant solution will be:

$92 \ g$ of $NO_{2(g)}$ and $32 \ g$ of $O_{2(g)}$ are taken in a rigid container. At a constant temperature,they react to produce $N_2O_{5(g)}$. The pressure in the container after the reaction is observed to be $\frac{7}{8} P$,where $P$ is the initial pressure of the gaseous mixture. The percentage yield of the reaction is ....$\%$

$NH_3$ is produced according to the following reaction: $N_{2(g)} + 3H_{2(g)} \rightarrow 2NH_{3(g)}$. In an experiment,$0.25 \ mol$ of $NH_3$ is formed when $0.5 \ mol$ of $N_2$ is reacted with $0.5 \ mol$ of $H_2$. What is the percentage yield (in $\%$)?