The mass of the solute that is being dissolved here is 7.15 g Na2CO3-10H2O (found on other sources). We use this amount to convert it to moles of Na2CO3. And by the dissociation of the salt in the solution, we can calculate the moles of Na+ ions are present.
Na2CO3 = 2Na+ + CO3^2-
7.15 g Na2CO3-10H2O ( 1 mol / 286 g) ( 1 mol Na2CO3 / 1 mol Na2CO3-10H2O ) ( 2 mol Na+ / 1 mol Na2CO3) = 0.05 mol Na+ ions present
