calculate the pH during the titration of 100 mL of .200 M HCl with .400 M NaOH after 0,25, 50, and 75 mL NaOH have been added
This is too long and too detailed to post; in general what you do is start with mols HCl, add x mols NaOH, subtract mols to see mols HCl unreacted, then M = mols/L (remember this is total volume which is mL HCl to start plus mL NaOH added). Then pH = -log(H^+)
I can expand on individual questions if you get stuck one of them. Post your work.