Chemistry - Stoichiometry Question with Solution | TestHub

Question Explanation: Determining total moles in a mixture based on thermal decomposition.

Concept: Stoichiometry of Carbonates and Bicarbonates.

Solution: Let moles of ${\mathrm{CaCO}}_3,{\mathrm{MgCO}}_3$, and ${\mathrm{NaHCO}}_3$ be $\mathrm{a},\mathrm{b}$, and c . Heating decomposes them; ${\mathrm{NaHCO}}_3$ produces ${\mathrm{Na}}_2{\mathrm{CO}}_3,{\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{CO}}_2$, while ${\mathrm{CaCO}}_3$ and ${\mathrm{MgCO}}_3$ produce oxides and ${\mathrm{CO}}_2$. The mass loss is the sum of gases evolved. The residue reacting with HCl gives ${\mathrm{CO}}_2$ only from ${\mathrm{Na}}_2{\mathrm{CO}}_3$.

$\left[{\mathrm{CaCO}}_3+{\mathrm{MgCO}}_3+{\mathrm{NaHCO}}_3\right]=26.8\mathrm{g}$

$a$ mole $b$ mole $c$ mole

$100a+84b+84c=\mathrm{26.8.}\dots \left(i\right)$

${\mathrm{CaCO}}_3(\mathrm{s})\to \mathrm{CaO}(\mathrm{s})+{\mathrm{CO}}_2(\mathrm{g})$

a - -

• - a a
• ${\mathrm{MgCO}}_3(\mathrm{s})$ $\to$ $\mathrm{MgO}(\mathrm{s})$ $+{\mathrm{CO}}_2(\mathrm{g})$
• b - -
• - b b
• $2{\mathrm{NaHCO}}_3(\mathrm{s})$ $\to$ ${\mathrm{Na}}_2{\mathrm{CO}}_3(\mathrm{s})$ $+{\mathrm{H}}_2\mathrm{O}(\mathrm{g})+C{O}_2\left(g\right)$
• c - - -
• - $\mathrm{c}/2$ $\mathrm{c}/2$ $\mathrm{c}/2$
• Loss in weight $=44\mathrm{a}+44\mathrm{b}+32\mathrm{c}=11.9\dots \left(ii\right)$ ${\mathrm{Na}}_2{\mathrm{CO}}_3+\mathrm{HCl}\to 2\mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$
• c/2
• $\frac{c}{2}=\frac{1.12}{22.4}=0.05$
• $\mathrm{c}=0.1$ mole
• On solving (1) & (2)
• $\mathrm{a}=0.1,\mathrm{b}=0.1$
• $(0.1+0.1+0.1)\times 10=3$