how does CO2 affect the titer of a solution of NaOH ?

Answer 1

It depends upon the indicator used for the titration. If methyl orange or methyl red is used there is no effect.
2NaOH + CO2 --> Na2CO3 + H2O

2NaOH makes one Na2CO3 and in using m.o. or m.r. you titrate all of the carbonate as
CO3^2- + 2H^+ --> H2O + CO2

If phenolphthalein is used as an indicator, then the CO3^2- is titrated only "halfway" as in CO3^- + H^+ ==> HCO3^-; therefore, only half the carbonate formed is titrated and the titre is smaller.