Question

5. An element X, in which the difference in electrons present in valence shell and penultimate shell is equal to the number of electrons present in the first shell, undergoes combustion to form Y. Y turns lime water milky due to formation of Z. Identify X, Y and Z.

• A. $\mathrm{C},{\mathrm{CO}}_2,{\mathrm{CaCO}}_3$
• B. $\mathrm{S},{\mathrm{SO}}_2,{\mathrm{CaSO}}_3$
• C. $\mathrm{S},{\mathrm{SO}}_2,\mathrm{Ca}{\left({\mathrm{HSO}}_3\right)}_2$
• D. $\mathrm{C},{\mathrm{CO}}_2,\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2$

Answer 1

The correct option is B

$\mathrm{S},{\mathrm{SO}}_2,{\mathrm{CaSO}}_3$

Explanation:

Part 1: Identifying X and Y:

• Element X is Sulphur $\left(\mathrm{S}\right)$ and its atomic number is 16. So its electronic configuration is 2,8,6.
• It is given that the difference in electrons present in the valence shell and penultimate shell of element X is equal to the number of electrons present in the first shell i.e. $8-6=2$
• Now, Sulphur undergoes combustion to form Y which is Sulphur dioxide$\left({\mathrm{SO}}_2\right)$.
• The reaction involved is: $\underset{\mathrm{Sulphur}\left(\mathrm{X}\right)}{\mathrm{S}}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2}\to \underset{\mathrm{Sulphur}\mathrm{dioxide}\left(\mathrm{Y}\right)}{{\mathrm{SO}}_2}$

Part 2: Identifying Z:

• It is also given that Y turns lime water milky. The chemical name of lime water is Calcium hydroxide$\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$.
• When lime water reacts with Sulphur dioxide, the following reaction takes place:
• $\underset{\mathrm{Y}}{{\mathrm{SO}}_2}+\underset{\mathrm{Lime}\mathrm{water}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2}\to \underset{\mathrm{Calcium}\mathrm{sulphite}\left(\mathrm{Z}\right)}{{\mathrm{CaSO}}_3}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}}$
• The Calcium sulphite formed is milky white in colour.

Hence, option (B) is the correct answer. Therefore, X is$\mathrm{S}$, Y is ${\mathrm{SO}}_2$ and Z is${\mathrm{CaSO}}_3$.