In which of the substances the mass fraction of oxygen is higher: in sand SiO2, in marble CaCO3.

Let’s calculate the mass of one mole of SiO2 sand and CaCO3 marble:
M SiO2 = 28 + 16 x 2 = 60 grams / mol;
M CaCO3 = 40 + 12 + 16 x 3 = 100 grams / mol;
The molar mass of two oxygen atoms O2 = 16 x 2 = 32 grams / mol.
The molar mass of three oxygen atoms O3 = 16 x 3 = 48 grams / mol.
Let us find the mass fraction of oxygen in one mole of SiO2:
K1 = 32/60 = 0.533 = 53.3%;
Let us find the mass fraction of oxygen in one mole of CaCO3:
K2 = 48/100 = 0.480 = 48%;
The mass fraction of oxygen in SiO2 sand is higher than in CaCO3 marble.