Question

25.3 g of sodium carbonate, Na${}_2$CO${}_3$ is dissolved in enough water to make 250 mL of solution. If sodium carbonate dissociates completely, the molar concentration of sodium ion, Na${}^{+}$ and carbonation, $CO{{}_3}^{2-}$ are respectively:
[Molar mass of Na${}_2$CO${}_3$ = 106 g mol${}^{-1}$]

• A. 0.955 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 0.477 M

Answer 1

The correct option is B 1.910 M and 0.955 M

As we know that,

$Molarity=\frac{moles}{Volume}$

$Moles=\frac{mass}{\text{molar mass}}$

Given:-

$\text{mass of}{Na}_{2}C{O}_3=25.3g$

$\text{Molar mass of}{Na}_{2}C{O}_3=106g$

$\text{Vol of sol.}=250mL=0.25L$

$\text{moles of}{Na}_{2}C{O}_3=\frac{25.3}{106}=0.2386$

$\text{Molarity of}{Na}_{2}C{O}_3=\frac{0.2386}{0.25}=0.955M$

When ${Na}_{2}C{O}_3$ dissociates, it gives 2 moles of ${Na}^{+}$ ion and 1 mole of ${C{O}_3}^{2-}$ ion.

$\therefore$ Molar concentration of sodium ion, $\left({Na}^{+}\right)=2\times 0.955=1.910M$

Molar concentration of carbonate ion, $\left({C{O}_3}^{2-}\right)=1\times 0.955=0.955M$