[vanderwaals]

25.0 mL of 0.10 M HCl solution is titrated with 0.10 M of NaOH solution. The final pH of the solution
is 12.00. How many mL of NaOH solution are added?
A. 25.0 B. 27.8 C. 55.6 D. 50.0

The answer is (C) 55.6 mL, but I keep getting 30.6 mL when I do the problem for some reason. I calculated that the concentration of OH- has to be 0.01 M for the pH to be 12.00, so ((the volume of NaOH x 0.10 M) - (total moles of HCl)) / (the total volume) must equal 0.01 M, and this only seems to work when 30.6 mL of NaOH is added. What am I doing wrong?

[XeLa.]

I get the same answer. I don't understand their logic. The final volume of the solution should be 55.6 mL, however, the volume of added 0.1 M NaOH solution should be 30.6 mL. Assuming, of course, that the solutions are additive.

[Borek]

Obvious mistake, 30.6 mL is a correct answer.

After adding 55.6 mL of the NaOH solution pH is 12.54

[vanderwaals]

Cool thanks!!