Answer :
To find the concentration of hydrogen ion from the pOH we need two important informations, first is that pOH + pH = 14, this is a constant number, therefore, the pH in our question needs to be pH = 14 - 2.85
pH = 11.15, now we have the pH for the question, now we can use to find the concentration of H+, which represents the acidity of a solution.
In order to find which was the correct option I have used the "pH = -log [H+]" formula, which tells us that the pH is equal to the negative log of the concentration of hydrogen ions.
a. pH = -log [1.38*10^-3] =
pH = 2.86
b. pH = -log [7.24*10^-12]
pH = 11.14
c. pH = -log [7.24*10^2]
pH = -2.86
d. pH = -log [3.50*10^-15]
pH = 14.4
From this data, we can see that letter B is the correct one
Another trick that helped me know the answer before the calculations, was to have in mind that the pOH was low, therefore the pH must be a high value, and therefore the concentration of H+ has to be a really small concentration, the one that fitted best was letter B