Which of the following is formed when calcium combines with oxygen?

$1 \ g$ of sodium hydroxide was treated with $25 \ mL$ of $0.75 \ M \ HCl$ solution. The mass of sodium hydroxide left unreacted is equal to:

According to the reaction $2A + B \to C$,$8 \ mol$ of $A$ and $5 \ mol$ of $B$ will produce how many moles of $C$?

Consider the following reaction occurring in the blast furnace: $Fe_3O_{4(s)} + 4CO_{(g)} \rightarrow 3Fe_{(l)} + 4CO_{2(g)}$. '$x$' kg of iron is produced when $2.32 \times 10^3 \ kg$ of $Fe_3O_4$ and $2.8 \times 10^2 \ kg$ of $CO$ are brought together in the furnace. The value of '$x$' is $.............$ (nearest integer). Given: Molar mass of $Fe_3O_4 = 232 \ g \ mol^{-1}$,Molar mass of $CO = 28 \ g \ mol^{-1}$,Molar mass of $Fe = 56 \ g \ mol^{-1}$.

In the reaction,$4NH_{3(g)} + 5O_{2(g)} \to 4NO_{(g)} + 6H_{2}O_{(g)}$,when $1 \ mole$ of ammonia and $1 \ mole$ of $O_{2}$ are made to react to completion:

$3O_2 + 2N_2 \to 2N_2O_3$
$9 \ mol$ of $O_2$ and $14 \ mol$ of $N_2$ are allowed to react. When $3 \ mol$ of $O_2$ remains unreacted,how many moles of $N_2O_3$ would have been produced?