The amount of a substance can be converted into grams of that substance by using the molar mass. It is a physical property of a substance which describes the mass per mole of the substance. The molar mass of Na2CO3 is 105.99 g/ mol. Multiplying the given amount of Na2CO3 with the molar mass will yield to the mass of the substance.
0.650 x 105.99 = 68.89 g Na2CO3