$$25.3\,g$$  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, molar concentration of sodium ion, $$N{a^ + }$$ and carbonate ion, $$CO_3^{2 - }$$  are respectively ( Molar mass of $$N{a_2}C{O_3} = 106\,g\,mo{l^{ - 1}}$$     )

Solution :
$$\eqalign{ & {\text{Molarity}} \cr & = \frac{{{\text{Number of moles of solute}}}}{{{\text{Volume of solution (in }}mL{\text{)}}}} \times 1000 \cr & = \frac{{25.3 \times 1000}}{{106 \times 250}} \cr & = 0.9547 \approx 0.955\,M \cr} $$
$$N{a_2}C{O_3}$$  in aqueous solution remains dissociated as
$$\mathop {N{a_2}C{O_3}}\limits_x \rightleftharpoons \mathop {2\,N{a^ + }}\limits_{2x} + \mathop {CO_3^{2 - }}\limits_x $$
Since, the molarity of $$N{a_2}C{O_3}$$  is $$0.955\,M,$$  the molarity of $$CO_3^{2 - }$$  is also $$0.955\,M$$  and that of $$N{a^ + }$$  is $$2 \times 0.955 = 1.910\,M$$