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Boiling this is done to remove ALL carbon dioxide (as gas) from the solution, before titrating the HCl-excess 'back' with titrant sodium hydroxide. Otherwise CO2 is disturbing the reaction at the end point of this backtitration at pH = 7 to 8 indicated by phenolphtaleine)Na2CO3.xH2Oaq + 2HClaq --> 2NaClaq + H2Oliq + CO2(gas) + H+aq,(excess)(Don't forget to cool down after boiling)OH- + H+(excess) ==> H2OOH- + CO2 |-o-| 'nill'(= no reaction of CO2 to form HCO3-, which is formed at pH = 7 to 8)
2NaOHaq + 2HClaq --> 2NaClaq + H2Ol is the perfect balanced equatiion,except the solubility (aq) of NaCl, so it is notNaCls but NaClaq