Blood plasma is maintained at a $\mathrm{pH}$ of $7.4$ largely by the

$\mathrm{pH}$ of a solution obtained on mixing $50\mathrm{ml}$ of $0.1\mathrm{M} \mathrm{NaCN}$ and $50\mathrm{ml}$ of $0.2\mathrm{M} \mathrm{HCl}$ is

Given $({\mathrm{pK}}_{\mathrm{a}}$ for $\mathrm{HCN}=9.40)$

How many of the following reactions are homogeneous reversible reactions?

($1$) ${\mathrm{CH}}_3{\mathrm{COOH}}_{\left(\mathrm{l}\right)}+{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OH}}_{\left(\mathrm{l}\right)}\rightleftharpoons {\mathrm{CH}}_3{\mathrm{COOC}}_2{{\mathrm{H}}_5}_{\left(\mathrm{l}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$

($2$) ${\mathrm{C}}_{\left(\mathrm{s}\right)}+{{\mathrm{CO}}_2}_{\left(\mathrm{g}\right)}\rightleftharpoons 2{\mathrm{CO}}_{\left(\mathrm{g}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}$

($3$) ${{\mathrm{H}}_2}_{\left(\mathrm{g}\right)}+{{\mathrm{CO}}_2}_{\left(\mathrm{g}\right)}\rightleftharpoons {\mathrm{CO}}_{\left(\mathrm{g}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}$

($4$) ${\mathrm{CO}}_{\left(\mathrm{g}\right)}+{{\mathrm{Cl}}_2}_{\left(\mathrm{g}\right)}\rightleftharpoons {{\mathrm{COCl}}_2}_{\left(\mathrm{g}\right)}$

($5$) ${\mathrm{NH}}_4{\mathrm{HS}}_{\left(\mathrm{s}\right)}\rightleftharpoons {{\mathrm{NH}}_3}_{\left(\mathrm{g}\right)}+{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{g}\right)}$

($6$) ${{\mathrm{CaCO}}_3}_{\left(\mathrm{s}\right)}\rightleftharpoons {\mathrm{CaO}}_{\left(\mathrm{s}\right)}+{{\mathrm{CO}}_2}_{\left(\mathrm{g}\right)}$

($7$) ${{\mathrm{N}}_2}_{\left(\mathrm{g}\right)}+{{\mathrm{O}}_2}_{\left(\mathrm{g}\right)}\rightleftharpoons 2{\mathrm{NO}}_{\left(\mathrm{g}\right)}$

($8$) ${{\mathrm{CO}}_2}_{\left(\mathrm{g}\right)}+{\mathrm{C}}_{\left(\mathrm{s}\right)}\rightleftharpoons 2{\mathrm{CO}}_{\left(\mathrm{g}\right)}$

($9$) ${{\mathrm{SO}}_2}_{\left(\mathrm{g}\right)}+{{\mathrm{NO}}_2}_{\left(\mathrm{g}\right)}\rightleftharpoons {{\mathrm{SO}}_3}_{\left(\mathrm{g}\right)}+{\mathrm{NO}}_{\left(\mathrm{g}\right)}$

($10$) ${\mathrm{NO}}_{\left(\mathrm{g}\right)}+\frac{1}{2}{{\mathrm{Br}}_2}_{\left(\mathrm{l}\right)}\rightleftharpoons 2{\mathrm{NOBr}}_{\left(\mathrm{g}\right)}$