Question

25.3 g of sodium carbonate $\left(N{a}_{2}C{O}_3\right)$ is dissolved in enough water to make 250 mL of a solution. If it dissociates completely, the molar concentration of sodium ions $\left(N{a}^{+}\right)$ and the carbonate ions $\left(C{O}_3^{2-}\right)$ are respectively:
(Molar mass of $N{a}_{2}C{O}_3$ = 106 $gmo{l}^{-1}$)

• A. 0.956 M and 0.478 M
• B. 1.912 M and 0.956 M
• C. 2.122 M and 1.061 M
• D. 0.954 M and 0.477 M

Answer 1

The correct option is B 1.912 M and 0.956 M

Number of moles of $N{a}_{2}C{O}_3=\frac{\text{given mass (g)}}{\text{molar mass (g/mol)}}=\frac{25.3}{106}=0.239\text{molarity}=\frac{\text{moles of solute of}N{a}_{2}C{O}_3}{\text{total volume of solution in mL}}\times 1000\text{molarity}=\frac{0.239}{250}\times 1000=0.956M$
According to the reaction: $N{a}_{2}C{O}_3\to 2N{a}^{+}+C{O}_3^{2-}$
$1M$ $N{a}_{2}C{O}_3$ will dissociate to $2M$ of $N{a}^{+}$ and $1M$ of $C{O}_3^{2-}$
0.956 M $N{a}_{2}C{O}_3$ dissociates to $2\times 0.956$ M $N{a}^{+}=1.912$ M
0.956 M $N{a}_{2}C{O}_3$ dissociates to 0.956 M of $C{O}_3^{2-}$