A white solid X, weakly alkaline in water (pH about 8), gives off a colourless gas that turns lime water milky and drops of a colourless liquid on gentle heating near 400 K. The residue Y is alkaline (pH about 11 at 0.1 M), and does not lose more mass on heating to 1000 K. Taking X to be the compound that fits these facts, the percentage of its mass lost on complete conversion to Y is (atomic masses: Na 23, C 12, O 16, H 1)

Answer: D

X is

NaHCO X_{3}

and Y is

Na X_{2} CO X_{3}

, because

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

fits the gas, the liquid and the stable alkaline residue. Mass lost per 168 g of X is

44 + 18 = 62

g, so the loss is

62/168 \times 100 = 36.9%

. The value 26.2% counts only

CO X_{2}

and 63.1% is the residue.