Molar mass of Na2CO3=(2×23)+(1×12)+(3×16)=106gmol−1
Now,
1 mole of Na2CO3 means 106g of Na2CO3
⇒0.5 mole of Na2CO3=0.5×106=53g Na2CO3
Further, 0.50M of Na2CO3 means 0.50 mol of Na2CO3 in 1 L of water, which means 53g of Na2CO3 is present in 1L of water
0.5 mol is the amount but 0.50 Mis 0.50 mol/L i.e., it is not the amount but the concentration.