A white solid X imparts a brick-red colour to the flame. On strong heating up to about 1000 $^{\circ}$ C it first gives a gas that burns with a blue flame, leaving a residue that effervesces with dilute HCl; further heating liberates a second colourless gas that turns lime water milky, leaving a residue that forms an alkaline suspension with water. Identify X and the total moles of gas per mole of X evolved on heating to constant mass.

Answer: A

Brick-red flame means calcium. Loss of a gas burning with a blue flame (CO) leaving a carbonate points to the oxalate:

CaC X_{2} O X_{4} CaCO X_{3} + CO

and then

CaCO X_{3} CaO + CO X_{2}

. Total gas is 1 mol CO + 1 mol

CO X_{2}

= 2 mol per mole of X;

CaO

gives alkaline

Ca (OH) X_{2}

with water. Strontium would give a crimson flame.