I think is ok, just consider same molar ammount

I agree with Dr Jimenez. You should consider the presence of crystallisation water in the stoichiometric calculus for concentration. 

I disagree.  Even though the molarity will be the same the ionic strength of the solution will not (particularly if it is pH adjusted).  Depending on what this solution is used for the ionic strength can be crucial.

I know that molecular weight would be the determining value on the calculation of molarity or normality. What Mr. Bruce says about ionic strenght  is interesting. Sodium carbonate will be diluted in distilled water at a certain concentration (e.g. 7%). Could someone please try to explain it? Thank you.

Dear, it depends on the type of experiment and it's role in it. However you have to put the water molecules in consideration. If you prepared your solution with molarity or normality you will get the same effect for both type but the % does not consider the effects of water molecules. 

If the % concentration is w/v it may be easily transformed in molarity. Once you know the moles of the sodium carbonate anhydrous you may also know the amount of the monohydrated form which contain the same quantity of Na2CO3 and take it to the required volume obtaining a solution with the same molarity. If we consider for simplicity a 1 M solution and consider the electrolyte completely dissociated in both cases in one liter of solution you have 2 moles of sodium ions and one mole of carbonate anion. Since the ionic strength is due to the total number of ions in solution and the concentration of ions in the two solutions is the same I do not see the difference. So an explanation should be necessary to affirm the contrary.

Ok, either Na2CO3 anhydrous or monohydrate must be prepared at 1 M, both own the same quantity of sodium ions (2) and carbonate anion (1) giving them the same ionic strength. Thank you Dr. Venditti and everybody.