Question

$25.3g$ of sodium carbonate, $N{{a}_{2}}C{{O}_{3}}$ is dissolved in enough water to make 250 ml of solution . If sodium carbonate dissociates completely, the molar concentration of sodium ion , $N{{a}^{+}}$ and carbonation $CO_{3}^{2-}$ are respectively:
[Molar mass of $N{{a}_{2}}C{{O}_{3}}=106\,g\,mo{{l}^{-1}}$ ]
A. 0.55 M and 1.910 M
B. 1.910 M and 0.955 M
C. 1.90 M and 1.910 M
D. 0.477 and 0.477M

Answer 1

When sodium carbonate is dissolved in water, it forms carbonic acid and sodium hydroxide.
Sodium hydroxide is a strong base whereas carbonic acid is a weak acid.
The approximate pH value of carbonic acid is $5.6$
For this question, we are required to find molarity of the solution.

Complete step by step answer:
First of all,we will find the number of moles of sodium carbonate
$\left( N{{a}_{2}}C{{O}_{3}} \right)\Rightarrow \dfrac{25.3}{106}=0.239$
[Molar mass of $N{{a}_{2}}C{{O}_{3}}=106 mo{{l}^{-1}}$ given in question]
Now, molarity of solution $\Rightarrow \dfrac{0.239}{0.25}=0.95mo{{l}^{-1}}$
Dissociation equation for $N{{a}_{2}}C{{O}_{3}}$ is given by
$N{{a}_{2}}C{{O}_{3}}\xrightarrow{{}}2N{{a}^{+}}+C{{O}_{3}}^{2-}$
We can see that the concentration of sodium is twice.
Therefore, concentration of $C{{O}_{3}}^{2-}=0.956m$
And concentration of $N{{a}^{+}}=2\times 0.956$
$1.912M$
So, the correct answer is “Option B”.

Note: 1. Carbonic acid $\left( {{H}_{2}}C{{O}_{3}} \right)$ can dissolve limestone, which leads to the formation of calcium bicarbonate $\left[ Ca{{\left( HC{{O}_{3}} \right)}_{2}} \right]$. Carbonic acid helps in the transformation of carbon – dioxide out of the body. It is also used in the production of soft drinks, artificially carbonating separately water and other bubbly beverages.
2. Carbonic acid salts are called bicarbonate ( carbonate of hydrogen or carbonate).