Hard to answer.
1. You must have Keq of ascorbic acid to determine pH.
2. Hardly have I listened using a weak acid to neutralize a strong base. It is recommended to use strong acid since "neutralize" almost means to make a 7-pH solution. In this case, if you use 350mg weak acid to react sufficiently with NaOH, we scarely have a 7-pH solution but a weak base solution. However, If you have Keq of weak acid, you can calculate the amount of this to add to NaOH solution to make a nearly 7-pH solution
3.Similarily, we need Keq1 and Keq2 of H2CO3.