Final answer:
To calculate the pH at different points during the titration of nitric acid with sodium hydroxide, one must use stoichiometry and knowledge of strong acid-base reactions.
Step-by-step explanation:
Initial pH: Before any NaOH is added, all we have is the nitric acid solution. The concentration of hydrogen ions, [H+], is equal to the concentration of the nitric acid, which makes the initial pH = -log(0.125) = 0.903.
pH at equivalence point: At this point, all the original HNO3 has been neutralized by NaOH. The equivalence point occurs when the moles of HNO3 are equal to the moles of NaOH. Based on their concentrations and volumes, we find the volume of NaOH needed to reach the equivalence point using the equation V1N1=V2N2, giving us 20.0 mL * 0.125 M / 0.150 M = 16.7 mL. At equivalence, the solution is neutral, so pH = 7.
pH before equivalence point: Before reaching the equivalence point, there will be excess HNO3, and we can calculate the pH based on the remaining concentration of HNO3 after partial neutralization with NaOH.
pH after equivalence point: Beyond the equivalence point, NaOH is in excess, and we can calculate the pH based on the concentration of OH- ions remaining in the solution. For calculating pH at different points of titration, we'll use stoichiometry to find the concentration of H+ or OH- ions, and then convert that to pH or pOH, and finally to pH if needed.