At \( 25\,^{\circ}\mathrm{C} \), the concentration of \( \mathrm{H}^{+} \) ions in \( 1.00 \times 10^{-3} \) \( \mathrm{M} \) aqueous solution of a weak monobasic acid having acid dissociation constant \( (K_a) \) of \( 4.00 \times 10^{-11} \) is \( X \times 10^{-7} \) \( \mathrm{M} \). The value of \( X \) is __________.
Use: Ionic product of water \( K_w = 1.00 \times 10^{-14} \) at \( 25\,^{\circ}\mathrm{C} \)