Question

20.0 mL of a 0.18 M HCl solution are combined with 50.0 mL of a 0.085 M NaOH solution.

What is the pH of the resulting solution?

Answer 1

millimols HCl = mL x M = 3.60
mmols NaOH = approx 5 but you need a better answer than that.
.......HCl + NaOH ==> NaCl + H2O
I......3.6....5........0......0
C....-3.6...-3.6................
E......0...aprx 1.4........
So you have an excess of approx 1.4 millimols NaOH; the concn is M = mmols/mL = approx 1.4/70 mL = 0.02M
pOH = -log(OH^-) = -log(NaOH), then
pH + pOH = pKw = 14 and solve for pH. Post your work if you get stuck.