calculate the number of moles of sodium carbonate in one dm3

Answer 1

do you mean density of Na2CO3 of 1 dm^3 volume? if so, we first find its density. Since sodium carbonate only, assume that it is anhydrous (no water/hydrate). from density data found in some books, density of Na2CO3 is 2.54 g/cm^3.
then we get the corresponding mass for a volume of 1 dm^3. note that 1 dm^3 = 1000 cm^3, thus,
m = 2.54 g/cm^3 * 1000 cm^3 = 2540 g Na2CO3
then we calculate its molar mass. we get the masses of each element (Na, C and O) from periodic table:
Na = 23 , C = 12 , O = 16
mass of Na2CO3 = 2*23 + 12 + 3*16 = 106 g/mol
finally, we get the number of moles by dividing the given/calculated mass (the 2540 g) by the molar mass:
2540/106 = 23.9623 mol Na2CO3

hope this helps~ :)