For Cr$_2$O$_7^{2-}$ + 14H$^+$ + 6e$^-$ $\rightarrow$ 2Cr$^{3+}$ + 7H$_2$O; E$^\circ$ = 1.33 V. At [Cr$_2$O$_7^{2-}$] = 4.5 millimole, [Cr$^{3+}$] = 15 millimole, E is 1.067 V. The pH of the solution is nearly equal to