$12-7.$ Calculate $pC{o}^{2+}$ at each of the following points in the titration of $25\mathrm{.}00mL$ of $0\mathrm{.}02026MC{o}^{2+}$ by $0\mathrm{.}03855M$ EDTA at $pH6\mathrm{.}00$ :

$($a$)12\mathrm{.}00mL$; $($b$)V_e$; $($c$)14\mathrm{.}00mL.$