posted by fred .
more volume of NaOH means more moles of NaOH (volume * molarity = mole ; molarity=constant) , more moles of NaOH means more grams and mole of HCl and more grams and/or mole and/mass of M2CO3 (note that they are both on the same side of the reaction)
now note that if the mass of M2CO3 is known and u want to calculate the molar mass of it, since mass / mole = molar mass, so the molar mass will be smaller than the real one . . but if the mole are known and you want to calculate the molar mass again since mass/molar = molar mass, this time the calculatod moalr mass will be higher then the actual one
I don't see a question in all of this. Frankly, it's confusing, too.