Answer: -
Baking soda having a pH of 9, has the maximum number of hydroxide ions.
Explanation: -
More the pH of a solution, the more basic it is and more the number of hydroxide ions it has.
From the question we find that
baking soda has a pH = 9
milk has a pH = 6
tomato juice has a pH = 3.5
vinegar has a pH = 2
Thus among them baking soda has the highest pH.
So baking soda is the most basic and has the maximum number of hydroxide ions.
