Ostwald's process for the manufacfure of nitric acid involves the reaction,

$4{\mathrm{NH}}_3\left(\mathrm{g}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\rightleftharpoons 4\mathrm{NO}\left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

                                              $\mathrm{\Delta H}=+\mathrm{Q} \mathrm{kJ}$

Which of the following factors will not affect the concentration of ${\mathrm{NH}}_3$ at equilibrium?