In the mixture of NaHCO3 and Na2CO3 volume of a given HCl required is 'x' ml with phenolphthalein indicator and further 'y' ml required with methyl orange indicator. Hence volume of HCl for complete reaction of NaHCO3 is
(y - x)
Na2CO3+HCl→NaHCO3+HCl
NaHCO3+HCl→NaCl+H2O+CO2
Let ‘a’ milli equivalents of acid used at the first end point.
Milli equivalents acid = x = 12 milli equivalents of Na2CO3
Let ‘b’ milli equivalents of acid used at the second end point.
Milli equivalents acid = y = 12 milli eq. of Na2CO3 + 1 milli eq. of NaHCO3
milli equivalents used by Na2CO3 = (2x)
milli equivalents of NaHCO3 = (y - x)