Question

A $100\%$ pure sample of a divalent metal carbonate weighing $2\, g$ on complete thermal decomposition releases $448 \,cc$ of carbon dioxide at STP. What is the equivalent mass of the metal?

Answer 1

Divalent metal carbonate means $M CO_3$.

$448 \,cc$ evolves from $2g$
$1\, cc $ evolves from $ \frac{2}{448} g$
$22400 \,cc$ will evolve from $\frac{2}{448} \times 22400\,g = 100\,g$
i.e., $100\,g$ is the molecular weight of the carbonate.
Then $M$.Wt. of metal $= \underset{\text{(wt. of carbonate} CO_3^{2-})}{ 100 - (12 + 3 \times 16) = 40\, g}$
Equivalent weight $= \frac{\text{Molecular weight}}{\text{Valency}}$
$ = \frac{40}{2} = 20$