1. 10 ml H3PO4 (74 m/m%) + 11,6971g NaOH (87 m/m%) in 990 ml H2O
I'm assuming for this one you use the pH formula for an ampholyte bc you get: H3PO4 + NaOH → NaH2PO4 + H2O. Altho you do start with an acid (H3PO4) or does that not have anything to do with it?
Start with the stoichiometry, once you know what is present in the solution we will think what to do next.
Not sure what "11,6971g NaOH (87 m/m%)" means, this is rather ambiguous - either 11.6971 g of NaOH (then 87% info needs to be ignored), or 11.6971 g of NaOH solution
(then 87% is an important part of the data).
5g Na2CO3 + 250ml H2O
I'm assuming here you use the pOH formula of a weak base to get the pH
Yes, this is a base solution, so pOH is the way to go.