Balance the following chemical equations:
$(i)$ $Cu_2S + O_2 \to Cu_2O + SO_2$
$(ii)$ $Ca_3(PO_4)_2 + H_2SO_4 \to Ca(H_2PO_4)_2 + CaSO_4$

For the reaction $2Al + 3MnO \xrightarrow{\Delta} Al_2O_3 + 3Mn$; If $108 \ g$ of $Al$ and $213 \ g$ of $MnO$ are heated,then which of the following is incorrect?

$CaCO_{3(s)} + 2 HCl_{(aq)} \rightarrow CaCl_{2(aq)} + CO_{2(g)} + H_2O_{(l)}$
Consider the above reaction,what mass of $CaCl_2$ will be formed if $250 \ mL$ of $0.76 \ M \ HCl$ reacts with $1000 \ g$ of $CaCO_3$ (in $g$)?
(Given: Molar mass of $Ca, C, O, H$ and $Cl$ are $40, 12, 16, 1$ and $35.5 \ g \ mol^{-1}$,respectively)

The heat of neutralisation of a strong acid and a strong base is $13.7 \ kcal$. The heat released when $0.6 \ mole$ $HCl$ solution is added to $0.25 \ mole$ of $NaOH$ is: (in $kcal$)

$3O_2 + 2N_2 \to 2N_2O_3$
$9 \ mol$ 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?

For a given unbalanced reaction, $MnO_2 + HCl \rightarrow MnCl_2 + H_2O + Cl_2$, which is the limiting reagent if the initial amount for each reactant is $100 \ g$? [Molar masses: $MnO_2 = 86.9 \ g/mol$; $HCl = 36.5 \ g/mol$]