Calcium carbonate reacts with aqueous $\mathrm{HCl}$ to give ${\mathrm{CaCl}}_2$ and ${\mathrm{CO}}_2$ according to the reaction,
${\mathrm{CaCO}}_3\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$.
What mass of ${\mathrm{CaCO}}_3$ is required to react completely with $25 \mathrm{mL}$ of $0.75 \mathrm{M} \mathrm{HCl}$

Chlorine is prepared in the laboratory by treating manganese dioxide (${\mathrm{MnO}}_2$) with aqueous hydrochloric acid according to the reaction.

$4 {\mathrm{HCl}}_{\left(\mathrm{aq}\right)} + {\mathrm{MnO}}_{2\left(\mathrm{s}\right)}\to 2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)} + {\mathrm{MnCl}}_{2\left(\mathrm{aq}\right)} + {\mathrm{Cl}}_2 \left(\mathrm{g}\right)$

How many grams of $\mathrm{HCl}$ react with $5.0 \mathrm{g}$ of manganese dioxide?

Calculate the volume of $0.1\mathrm{N} {\mathrm{H}}_2{\mathrm{SO}}_4$ required to neutralize $200\mathrm{ml}$ of $0.2\mathrm{N} \mathrm{NaOH}$ solution.

It is an acid base neutralisation reaction.Hence, at the neutralisation point.

Number of equivalents of acid $=$ Number of equivalents of base.

Assign oxidation number to the underlined element in the following species:

$\mathrm{NaH}\underline{\mathrm{S}}{\mathrm{O}}_4$.

Assign oxidation number to the underlined element in the following species:

${\mathrm{H}}_4{\underline{\mathrm{P}}}_2{\mathrm{O}}_7$