A mixture of $Na X_{2} CO X_{3}$ (106 g/mol) and $NaHCO X_{3}$ (84 g/mol) is titrated with 0.100 M HCl. The phenolphthalein end point is reached after 15.0 mL and, after adding methyl orange, a further 35.0 mL is needed. The mass percentage of $NaHCO X_{3}$ in the mixture is:

Answer: B

Phenolphthalein volume converts only carbonate:

Na X_{2} CO X_{3}

= 0.100

\times

15.0 = 1.50 mmol (159 mg). The second stage needs carbonate-derived bicarbonate (15.0 mL) plus original bicarbonate, so

NaHCO X_{3}

= 0.100

\times

(35.0 - 15.0) = 2.00 mmol (168 mg). Percentage of

NaHCO X_{3}

= 168/327

\times

100 = 51.4 %.