Option A: baking soda, pH=9
The number of hydroxide ions or concentration of hydroxide ions depends on pOH of solution as follows:
\(pOH=-log[OH^{-}]\)
This means more the value of pOH, smaller will be the number of hydroxide ions.
Thus, solution with smaller value of pOH, will have greatest number of hydroxide ions.
Also, pH and pOH are related to each other as follows:
\(pOH=14-pH\)
Calculating pOH from all the given pH values,
pOH of baking soda:
pOH=14-9=5
pOH of milk:
pOH=14-6=8
pOH of tomato juice:
pOH=14-3.5=10.5
pOH of vinegar:
pOH=14-2=12
here, baking soda has smallest pOH value, thus, it will have greatest number of hydroxide ions.
Also, more the basic is solution, more it will have hydroxide ions, or more is the value of pH more the solution is basic. The pH of baking soda is maximum that is 9 thus, it is most basic among given options and contain greater number of hydroxide ions.
