Why did you have to remove co2 from your partially neutralized stomach acid before the titration?

Answer is: carbon dioxide (CO₂) can affect the end point of the titration.

Carbon dioxide, or carbon(IV) oxide, reacts with water and form carbonic acid, which can affect the end point of the titration, because more base is needed for neutralization of partially neutralized stomach acid.

Balanced chemical reaction of carbon dioxide and water: CO₂ + H₂O → H₂CO₃.

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.