Question

A box of washing soda has $1.43\text{kg}$ of $N{a}_{2}C{O}_3.10H_{2}O$ (molar mass = $286\text{g/mol}$). The box was sent to the godown and it weighed only $1.34\text{kg}$ on arrival. An independent analyst says that the box contains the correct mass of sodium carbonate, but that it has lost the water of crystallization. What is the composition of the washing soda received by the godown?

• A. $N{a}_{2}C{O}_3.1.3H_{2}O$
• B. $N{a}_{2}C{O}_3.1.7H_{2}O$
• C. $N{a}_{2}C{O}_3.8.3H_{2}O$
• D. $N{a}_{2}C{O}_2.8.9H_{2}O$

Answer 1

The correct option is C $N{a}_{2}C{O}_3.8.3H_{2}O$

Number of moles in $1430\text{g}$ of $N{a}_{2}C{O}_3.10H_{2}O$ is $\frac{1430}{286}$
Let the composition of sodium carbonate received be, $N{a}_{2}C{O}_3.x{H}_{2}O$. This weighs 1340 g
$\frac{1430}{286}\times (106+18x)=1340$
$\Rightarrow x=9$
where molar mass of anhydrous $N{a}_{2}C{O}_3$ is $106\text{g/mol}$