(i) $\mathrm{A} + \mathrm{Na}_{2}\mathrm{CO}_{3} \longrightarrow \mathrm{B} + \mathrm{C}$ (ii) A $\xrightarrow{\mathrm{CO}_2}$ (Milky) C The chemical formula of A and B are respectively
ANaOH and $\mathrm{Ca(OH)_2}$
B$\mathrm{Ca(OH)_2}$ and NaOH
CNaOH and CaO
DCaO and $\mathrm{Ca(OH)_2}$
Answer & Solution
Correct answer: B. $\mathrm{Ca(OH)_2}$ and NaOH
From statement $\mathrm{A} \xrightarrow{\mathrm{CO}_2} \mathrm{C}$ with $\mathrm{C}$ described as milky, $\mathrm{A}$ must be lime water and $\mathrm{C}$ must be the white precipitate formed when carbon dioxide is passed through it.
$$\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O}$$
So $\mathrm{A = Ca(OH)_2}$ and $\mathrm{C = CaCO_3}$.
Now use equation $(i)$.
$$\mathrm{Ca(OH)_2 + Na_2CO_3 \rightarrow 2NaOH + CaCO_3}$$
Hence $\mathrm{B = NaOH}$.
So the pair is $\mathrm{A = Ca(OH)_2}$ and $\mathrm{B = NaOH}$. Rechecking the options, this matches option $\mathrm{(B)}$.
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