I’m having trouble filling out these calculations on this sheet. I did the work but am having serious doubts.

I’m having trouble filling out these calculations on this sheet. I just need help with 1 trial then I will be able to to the other 7 trials. I did the work but am having serious doubts. I attached the sheet I need to complete, the lab data (in which we added 10 mL so the total volume per trial is 20mL instead of 10 mL), and my work. I need someone to look over the calculations. I’m stuck on how to calculate the initial I- molar concentration (6)