Asample of black coffee was measured to have [h3o+] = 7.94 × 10−6. what is true about this solution?

Being given the hydronium (or simply the hydrogen) ion concentration (molarity), one of the easiest properties of the solution to determine is its pH. The pH of a solution indicates the acidity (or basicity) of a given solution. The typical pH range of solutions is from 1 to 14, with 7 being neutral, 1 to 6 being acidic, and 8 to 14 being basic. 

To determine the pH of a solution, the negative log (-log) of the hydronium/hydrogen ion concentration (molarity) of the solution is calculated. Thus (assuming that the given hydrogen ion concentration is in molarity (mol/L)),

             pH = -log()          or          -log(
             pH = -log (7.94 × 10−6 mol/L) 
             pH = 5.13

Because the pH is less than 7 (within the range 1 to 6), then the black coffee solution is acidic. 

They are all full and don't need any more electrons in there outer shell.