How many grams of MgCO3 contain 24.00 g of oxygen? (The molar mass of MgCO3 is 84.30 g mol–1)
Mass of oxygen atom = 16 g
Mass of oxygen in 84.30 g of MgCO3 = 3 x 16 = 48 g
So, 24 g of oxygen will be in 42.15 g of MgCO3.
The correct option is A.