Describe how you would prepare 0.05 mol.dm-3 sodium carbonate in a total volume 250 mls.

Answer 1

For a 0.05 M (that's 0.05 mol/dm^3) Na2CO3 in 250 mL, how many mols Na2CO3 is that? That's mols = M x L = 0.05 M x 0.250 L = 0.0125 mols.
Then mols = grams/molar mass. You know mols and molar mass, solve for grams. To prepare such a solution, you add the grams Na2CO3 to an empty (and dry) 250 mL volumetric flask. Add about 100 mL distilled (or deionized) water, swirl until Na2CO3 is dissolved, then add water to the mark on the volumetric flask. Stopper, mix thoroughly, label. Done.

Answer 2

Good and helpful