Question

the pH of the solution containing 0.1 M nitric acid and 0.1 formic acid is

Answer 1

0.1% formic acid should give pH≈2.7, so a little bit more than 0.1% should drop the pH to the target value of pH-2.5. As in the TFA strategy above, you should get more consistent results if you use a constant amount of acid rather than trying to adjust the pH using acid.