A 10.0 g mixture of $NaHCO X_{3}$ and $Na X_{2} CO X_{3}$ is heated strongly until the mass is constant and the solid loses 1.55 g. The percentage by mass of $NaHCO X_{3}$ in the mixture is (Na 23, C 12, O 16, H 1)

Answer: C

Only

NaHCO X_{3}

decomposes:

2 NaHCO X_{3} Na X_{2} CO X_{3} + CO X_{2} + H X_{2} O

, with 62 g lost per 168 g. Mass of

NaHCO X_{3}

= 1.55 \times 168/62 = 4.20

g, which is 42.0% of 10.0 g. The value 15.5% is simply the mass loss as a percentage.