Question

When $10g$ of $90\%$ pure limestone is heated, the volume of $C{O_2}$(in litres) liberated at STP is:
A. $22.4\;L$
B. $2.24\;L$
C. $20.16\;L$
D. $2.016\;L$

Answer 1

Limestone is a carbonate rock which originated from the skeletal remains of marine organisms. It consists of calcium carbonate which has a chemical formula $CaC{O_3}$. It generally exists in sedimentary and crystalline form. It is also found as the main component of eggshells, seashells and pearls.

Complete answer:
As per conditions given in the question, the reaction proceeds as follows:
$CaC{O_3} \rightleftharpoons CaO + C{O_2}$
Now, it is given that $10g$ of $90\%$ pure limestone is heated which means the mass of calcium carbonate heated is as follows:
Mass of calcium carbonate $= 10 \times \dfrac{{90}}{{100}} \Rightarrow 9g$
Now, we know that the number of moles of any compound is the ratio of its mass reacting to its molar mass. Therefore, number of moles of calcium carbonate for the given reaction will be as follows:
$n = \dfrac{{{\text{mass of calcium carbonate heated}}}}{{{\text{molar mass of calcium carbonate}}}}\;\;\; - (1)$
As per our calculations, mass of calcium carbonate heated $= 9g$
Molar mass of calcium carbonate $= 100\;gmo{l^{ - 1}}$
Substituting values in equation (1):
$\Rightarrow n = \dfrac{9}{{100}}$
$\Rightarrow n = 0.09\;{\text{moles}}$
Hence, the number of moles of calcium carbonate heated to liberate carbon dioxide $= 0.09$
Now, as per reaction the number of moles of carbon dioxide evolved can be calculated as follows:
$\because 1$ mole of $CaC{O_3}$ is heated to form $\Rightarrow 1$ mole of $C{O_2}$
$\therefore 0.09$ moles of $CaC{O_3}$ will be heated to form $\Rightarrow 1 \times 0.09 = 0.09$ moles of $C{O_2}$
We know that, at STP 1 mole of carbon dioxide occupies $22.4L$ of volume. Therefore, volume occupied by $0.09$ moles of carbon dioxide will be as follows:
$V = 22.4 \times 0.09$
$\Rightarrow V = 2.016\;L$
Hence, the volume of $C{O_2}$(in litres) liberated at STP is $2.016\;L$. Thus, option (D) is the correct answer.

Note:
It is important to note that the heating of calcium carbonate to above ${840^o}C$ to form calcium oxide or quicklime along with the liberation of carbon dioxide gas is known as thermal decomposition reaction or calcination reaction.