Question

how many grams of potassium hydroxide would need to be added to 400 mL of 0.150M HNO3 to raise its pH to 2.5

Answer 1

HNO3 + KOH ==> KNO3 + H2O

mols HNO3 = M x L = 0.06
pH = 2.5 means you want (H^+) to be 0.00316 or mols (H^+) = 0.00316 x 0.4L = 0.00126. That's mols (H^+) remaining which means you must neutralize the difference; i.e., 0.06 - 0.00126 = ?
Convert that to grams KOH.