Question

A solution is prepared by mixing 30 ml of a 0.2 M hydrochloric acid (HCl) with 20 ml of a 0.3 M potassium hydroxide solution (KOH), what will be the final pH? (Show your calculation to get full credit.) Hint: Determine whether hydrochloric acid and potassium hydroxide are strong or weak acids or bases.

Answer 1

The final pH is 7

Step-by-step explanation:

KOH + HCl ------> KCl + H2O

KOH ----> K+ + OH-

0,3 M 0,3 M + 0,3 M

HCl -----> H+ + Cl-

0,2 M 0,2M + 0,2M

In both cases they are strong acid and base because they dissociate completely; The initial molarity is the same, at the end of the dissociation.

As we have an acid next to a strong base, the final solution will have a neutralization reaction where the OH- of the base react with the H + of the acid to give water. The excess of OH- or H + defines whether the solution will have an acid or alkaline pH.

Molarity = moles / volume (L)

Moles of OH- = 0,3 M x 0,020 L = 0,006 moles

(remember volume in L, so 20mL = 0,020 L)

Moles of H+ = 0,2 M x 0,030 L = 0,006 moles

As we have the same quantity for OH- and H+, the solution will have neutral pH.