It remains to find the number of mmol of and HF. Initially, there are (20.0 mL)(.2 mmol/mL) or 4.0 mmol of HF present. The strong base potassium hydroxide dissociates almost completely in the solution. Therefore, after adding 10.0 mL of .1M KOH, there are (10.0 mL)(.1 mmol/mL) or 1.0 mmol of hydroxide ions present. The 1.0 mmol of hydroxide ions reacts with 1.0mmol of the hydrofluoric acid, reducing the amount of hydrofluoric acid to 3.0 mmol and creating 1.0 mmol of fluoride ions. Substituting these values into equation (17) yields
The pH value has increased with the addition of the base, as expected.
