for the titration of 25.0mL of 0.20M hydrofluoric acid with 0.20M NaOH, determine the volume of base added when pH is 2.85?

Well?

Perhaps people are reluctant to answer your question because mixing these chemicals is dangerous, and your questions sounds like homework. If you tell what you know about this reaction, you may get some help. I am not a chemist, and do not know the answer you seek.

Could you calculate the concentration of Hydrogen ion in the solution before the addition of base because this is the only way you could get the answer.

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Moderator Note

People are reluctant to answer the question because this is a homework question and we are not in the habit of doing other people's work for them. Moved to the appropriate forum.