Answer:
The correct answer is 12.6.
Explanation:
The formation of benzoate takes place when potassium hydroxide reacts with benzoic acid, due to the presence of a weak acid and its conjugated base the solution will act as a buffer. In the given question, the molarity of benzoic acid given is 0.8700 M and its volume is 60.5 ml. Therefore, the moles of benzoic acid will be,
Moles = molarity * volume of solution
= 0.8700 M * 60.5 ml = 52.365 m mol or 0.052365 moles
On the other hand, the molarity of KOH given is 0.3600 M and the volume given is 172 ml. Therefore, the moles of KOH added will be,
Moles = 0.3600 * 172 = 61.92 m moles or 0.06192 moles
Out of this 61.92 m mol, only 52.365 m mol of KOH will react with the benzoic acid. The moles of KOH, which remain unreactive is,
61.92 m moles - 52.365 m moles = 9.285 m moles or 0.009285 moles
The formula for calculating molarity is number of moles / volume of solution in liters
The total volume of the solution is 172 ml + 60.5 ml = 232.5 ml or 0.2325 L
The molarity of KOH will be,
Molarity = 0.009285 moles / 0.2325 L = 0.0395 M
The dissociation of KOH takes place completely to produce hydroxide ion.
pOH = -log[0.0395] = 1.4
pH + pOH = 14
pH = 14 - 1.4 = 12.6
