A 1.68 g sample of a white solid is heated strongly. A gas that turns lime water milky is evolved, and water droplets condense on the cool part of the tube. Atomic masses: Na 23, C 12, O 16, H 1. The solid and the mass of the residue (assuming complete decomposition) are

Answer: D

Water vapour together with

CO X_{2}

shows a hydrogencarbonate:

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

. Moles of

NaHCO X_{3}

= 1.68/84 = 0.0200, so

Na X_{2} CO X_{3}

= 0.0100 mol = 0.0100 x 106 = 1.06 g. The mass lost is 0.62 g, which is the distractor. Plain

Na X_{2} CO X_{3}

does not decompose and gives no water.