$250 \ mL$ 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) $(N_{A} = 6.022 \times 10^{23})$

What is the number of moles of water molecules required to prepare $n$ moles of methane from $n$ moles of methyl magnesium iodide?

Phosphoric acid is prepared commercially from phosphate ore as follows:
$Ca_3(PO_4)_2 + 3SiO_2 + 5C + 5O_2 + 3H_2O \to 3CaSiO_3 + 5CO_2 + 2H_3PO_4$
If $1.0 \ kg$ each of calcium phosphate and silica are used with an excess of $C$,$O_2$,and $H_2O$,what is the maximum quantity of phosphoric acid that can be produced (in $kg$)? (Atomic masses: $Ca = 40, P = 31, Si = 28, O = 16, H = 1$)

$2 \ L$ of $0.2 \ M \ H_2SO_4$ is reacted with $2 \ L$ of $0.1 \ M \ NaOH$ solution. The molarity of the resulting product $Na_2SO_4$ in the solution is $X \ mM$. Find the value of $X$. (Nearest integer).

Percentage loss in mass on heating a mixture of $Na_2CO_3$ and $CaCO_3$ containing equal mass of the two components will be ............. $\%$

When $81.0 \ g$ of aluminium is allowed to react with $128.0 \ g$ of oxygen gas,the mass of aluminium oxide produced in grams is $..........$ $(Nearest \ integer)$. Given: Molar mass of $Al$ is $27.0 \ g \ mol^{-1}$,Molar mass of $O$ is $16.0 \ g \ mol^{-1}$.