What does the group number of an element tell you about the electrons?

The group number tells you the total number of electrons present in the outermost orbit of an atom. In other words, group number tells you the number of valence electrons of an atom. For example, Let us consider group 1 of the Periodic table.

The pH of the a certain substance is the negative log of the concentration of hydroxide ion. 
                                pH = -log[H+]
Substituting,
                                  8.57 = -log[H+]
The value of [H+] from the equation is 2.69x10^-9 molar. Thus, the concentration of hydroxide ion is approximately equal to 2.69x10^-9 M.