Question

25.0 mL of a 0.20 mol/L solution of potassium hydroxide is mixed with 25.0 mL of a 0.15 mol/L solution of hydrobromic acid. What is the pH of the resulting solution?

Answer 1

KOH + HBr ==> KBr + H2O

millimols KOH = mL x M = 25.0 x 0.20 = 5.00
millimols HBr = 25.0 x 0.15 = 3.75
millimols KOH left over = 5.0 - 3.75 = 1.25
(KOH) remaining = millimoles/mL = 1.25/50.0 = ?
pOH = - log (KOH)
Then pOH + pH = pKw = 14. You know pKw and pOH, solve for pH.
Post your work if you get stuck.