Qualitative Analysis - Result Question 75
78. When $16.8 g$ of white solid, $X$ were heated, $4.4 g$ of acid gas $A$, that turned lime water milky was driven off together with $1.8 g$ of a gas $B$ which condensed to a colourless liquid.
The solid that remained, $Y$, dissolved in water to give an alkaline solution, which with excess barium chloride solution gave a white precipitate $Z$. The precipitate effervesces with acid giving of carbon dioxide. Identify $A, B$ and $Y$ and write down the equation for the thermal decomposition of $X$.
(1984,4 M)
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Solution:
$(X)$ is $\mathrm{NaHCO}_3(M=84)$. The reactions involved are :
$2 \mathrm{NaHCO}_3 \longrightarrow \mathrm{Na}_2 \mathrm{CO}_3(\mathrm{~s})+\mathrm{CO}_2(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}(\mathrm{g}) $
$\quad X \quad \quad \quad \quad \quad \quad \quad Y \quad \quad \quad \quad A \quad \quad \quad \quad \quad B $
$\quad 168 \mathrm{~g} \quad \quad \quad \quad 106 \mathrm{~g} \quad \quad\quad \quad 44 \mathrm{~g} \quad \quad \quad \quad 18 \mathrm{~g} $
$\quad 16.8 \mathrm{~g} \quad \quad \quad \quad 10.6 \mathrm{~g} \quad \quad \quad 4.4 \mathrm{~g} \quad \quad \quad \quad 1.8 \mathrm{~g} $
$CO_2 + Ca(OH)_2 \rightarrow \quad \mathrm{CaCO}_3 \downarrow+\mathrm{H}_2 \mathrm{O} $
$\mathrm{H}_2 \mathrm{O}(\mathrm{g})$ is condensed to liquid water.
$\underset{Y}{\mathrm{Na}_2 \mathrm{CO}_3}+\mathrm{BaCl}_2 \longrightarrow \underset{Z}{\mathrm{BaCO}_3}+2 \mathrm{NaCl} $
$\underset{Z}{\mathrm{BaCO}_3}+2 \mathrm{HCl} \longrightarrow \mathrm{BaCl}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 $
BETA
