A 12.5 g sample of impure limestone (the impurity is inert and non-volatile) is heated strongly and gives 2.24 L of CO2 measured at STP (molar volume 22.4 L/mol). The mass of solid residue left is (Ca 40, C 12, O 16)

JEE Advanced levelSingle correct optionDifficulty 3 of 3sb206

A 12.5 g sample of impure limestone (the impurity is inert and non-volatile) is heated strongly and gives 2.24 L of $CO X_{2}$ measured at STP (molar volume 22.4 L/mol). The mass of solid residue left is (Ca 40, C 12, O 16)

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

2.24/22.4 = 0.100

mol

CO X_{2}

means 0.100 mol

CaCO X_{3}

(10.0 g, 80.0% purity). Mass lost is

0.100 \times 44 = 4.40

g, so the residue is

12.5 - 4.40 = 8.10

g (CaO 5.60 g plus 2.50 g of impurity).