First, the applicable rxn:
Na2CO3 + 2HCl -> CO2 + 2NaCl + H2O
This tells us that the mole ratio of sodium carbonate to HCl is 1:1. Given the mass of the sodium carbonate and its mole wt (106), The reactant was present at 265 mg/106g/mole or about 2.5 millimole. So one-tenth of this, or...