When 100 g of sodium carbonate is heated, a basic oxide with a mass of …

Let’s write the reaction equation:
Na2CO3 = Na2O + CO2
Let’s write an expression to determine the amount of a substance:
ν = m / M
We have the amount of sodium carbonate and oxide substances are equal, judging by the reaction equation:

m (Na2CO3) / M (Na2CO3) = m (Na2O) / M (Na2O)
Let’s define the molar masses:
M (Na2CO3) = 22.9 * 2 + 12 + 16 * 3 = 105.8 g / mol
M (Na2O) = 22.9 * 2 + 16 = 61.8 g / mol
Let us express the mass of the oxide from the previous formula:

m (Na2O) = m (Na2CO3) * M (Na2O) / M (Na2CO3)
Substitute the numerical values:
m (Na2O) = 100 * 61.8 / 105.8 = 56

Answer: the mass of the oxide is 56 g.