To calculate the hydronium ion concentration for a solution that has a pH of 10.61, you would do the following. Since pH=-log[H3O+]. Input the pH value and solve for [H3O+].
10.61=-log[H3O+]
-10.61=log[H3O+]
-10.61=log10[H3O+]
[H3O+]=10^-10.61
[H3O+]= 2.4547x10^-8.