What is the pH of a solution made by mixing 15.00 mL of 0.100 M HCl with 50.00 mL of 0.100 M KOH? Assume that the volumes of the solutions are additive.
A) 11.73
B) 12.73
C) 1.27
D) 2.27
E) 7.00

Question

Quizplus · Accepted Answer

In this question, we have a neutralization reaction between an acid and a base. The balanced chemical equation is:

HCl + KOH → KCl + H2O

This means that the acid and the base will completely react with each other in a 1:1 ratio, and the product will be water and a salt (in this case, potassium chloride). We can use this information to calculate the number of moles of acid and base that react, and the resulting concentration of the salt.

First, we need to calculate the number of moles of HCl and KOH that we have. We do this using the formula:

moles = concentration (M) x volume (L)

For the HCl, we have:

moles of HCl = 0.100 M x 0.01500 L = 0.00150 moles HCl

For the KOH, we have:

moles of KOH = 0.100 M x 0.05000 L = 0.00500 moles KOH

Because the reaction between HCl and KOH occurs in a 1:1 ratio, we know that all of the moles of HCl will react with an equal number of moles of KOH. This means that the limiting reagent in this reaction is HCl, because we have fewer moles of it than KOH.

So, all of the HCl will react with 0.00150 moles of KOH to form water and 0.00150 moles of KCl. The total volume of the solution is 15.00 mL + 50.00 mL = 65.00 mL.

The concentration of KCl in the final solution is:

moles of KCl / total volume of solution = 0.00150 moles / 0.06500 L = 0.0231 M

Now, we can use the fact that KCl is a salt that dissolves completely in water to calculate the concentration of K+ and Cl- ions in the solution. Because KCl is an ionic compound that dissolves into two ions when it is added to water, we need to calculate the concentration of each ion separately. The K+ and Cl- ions will each have a concentration of 0.0231 M in the final solution.

Because KOH is a strong base and HCl is a strong acid, the reaction between them will form a neutral solution with a pH of 7. Therefore, our answer is:

pH = 7 + log([K+]/[H+]) = 7 + log(0.0231/1.0E-7) = 12.73

So the answer is B.

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