Question

The compound $Na_2CO_3 . x \, H_2O$ has $50\% H_2O$ by mass. The value of "$x$ " is

• A. 4
• B. 5
• C. 6
• D. 7
• E. 8

Answer 1

Answer: (C)

(i) $\because$ Molar mass of $Na _{2} CO _{3}=106$ unit

$(23 \times 2)+12+(3 \times 16)$

and $CO _{3}^{2-}$ has two $(-)$ charge

While two $Na ^{+}$ ions are involved.

(ii) Number of moles (per unit charge) $=50 \%$ of $106$ is $106 / 2=53\, g$

$=\frac{53}{18} $

$=2.94$ mol

Total moles of $H _{2} O =2 \times 2.94 \equiv 5.89 \approx 6.00$