sblock Elements 2 Question 3
3. A hydrated solid $X$ on heating initially gives a monohydrated compound $Y$.
$Y$ upon heating above $373 \mathrm{~K}$ leads to an anhydrous white powder Z. $X$ and $Z$, respectively, are (2019 Main, 10 April II)
(a) baking soda and soda ash
(b) washing soda and soda ash
(c) baking soda and dead burnt plaster
(d) washing soda and dead burnt plaster
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Answer:
Correct Answer: 3. (c)
Solution:
- Baking soda $\left(\mathrm{NaHCO}{3}\right)$ is not a hydrated solid. Thus, $(X)$ is not baking soda. Thus, option (a) and (c) are incorrect. Dead burnt plaster $\left(\mathrm{CaSO}{4}\right)$ is obtained from gypsum via the formation of plaster of Paris.
$$ \underset{\text { Gypsum }}{\mathrm{CaSO}{4} \cdot 2 \mathrm{H}{2} \mathrm{O}} \xrightarrow[-1 / 2 \mathrm{H}{2} \mathrm{O}]{\stackrel{380-393 \mathrm{~K}}{\longrightarrow}} \mathrm{CaSO}{4} \cdot \frac{1}{2} \mathrm{H}{2} \mathrm{O} \xrightarrow[\text { Plaster of Paris }]{\stackrel{>393 \mathrm{~K}}{-\frac{1}{2} \mathrm{H}{2} \mathrm{O}}} \underset{\begin{array}{l} \text { Dead burnt plaster } \ \text { (anhydrous) } \end{array}}{\mathrm{CaSO}_{4}} $$
Therefore, the reaction takes place as follows :
$$ \underset{\text { Washing soda }}{\mathrm{Na}{2} \mathrm{CO}{3} \cdot 10 \mathrm{H}{2} \mathrm{O}} \xrightarrow[-9 \mathrm{H}{2} \mathrm{O}]{<373 \mathrm{~K}} \underset{\text { Monohydrate }}{\mathrm{Na}{2} \mathrm{CO}{3} \cdot \mathrm{H}{2} \mathrm{O} \xrightarrow[-\mathrm{H}{2} \mathrm{O}]{>37 \mathrm{~K}}} \underset{\text { Anhydrous white powder }}{\mathrm{Na}{2} \mathrm{CO}{3}} $$
$$ (X) \quad(Y) \quad \text { (soda ash) }(Z) $$
