Equal volumes of three basic solutions of 11,12 and 13 are mixed in a vesel. What will be the concentration in the mixture ?

Question

$\begin{array}{1 1} (a)\;1.11 \times 10^{-11}\;M \\(b)\;3.7 \times 10^{-12} \\ (c)\;3.7 \times 10^{-11}\;M\\ (d)\;1.11 \times 10^{-22}\;M \end{array} $