Question

42 g of a metallic carbonate $MC{O}_3$ was heated in a hard glass tube and $C{O}_2$ evolved was found to have 1120 mL of volume at STP. The Equivalent weight of the metal is :

• A. 12
• B. 24
• C. 18
• D. 15

Answer 1

The correct option is A 12

$22400$ mL$=1$mol of $C{O}_2=2$ Eq of $C{O}_2$

$11200$ mL$=1/2$ mol of $C{O}_2=1$Eq of $C{O}_2$ $=1$ Eq of $C{O}_3^{2-}$

Weight of metallic carbonate that would produce $1$ g equivalent or $22$ g or $11.2$ L of $C{O}_2$ at STP would be its Ew.

Ew of metallic carbonate $=\frac{4.2\times 11200}{1120}=42g$

Ew of metal $=$ Ew of $MC{O}_3$ - Ew of $C{O}_3^{2-}$ $=42-30=12$

$[\text{Ew of}C{O}_3^{2-}=\frac{60}{2}=30]$

Alternatively:
$22400mL=1$ mol of $C{O}_2=2EqC{O}_3^{2-}$

$11200mL\Rightarrow 1$ Eq of $C{O}_3^{2-}$

$\therefore 1120mL\Rightarrow \frac{1}{11200}\times 1120=0.1$ Eq of $C{O}_3^{2-}$

Eq of $MC{O}_3=$ Eq of $C{O}_3^{2-}$

$\frac{Weight}{Ew}=0.1Eq$

$\therefore 0.1=\frac{4.2}{EwofMC{O}_3}$

$\therefore Eq$ of $MC{O}_3=42$

$\therefore$ Eq of $M=Ew$ of $MC{O}_3-$ Ew of $C{O}_3^{2-}$$=42-30=12$