The chemical formula for quartz is SiO₂
Percentage by mass of oxygen = (2 x 16) x 100 / (28 + 2 x 16)
Percentage by mass of oxygen = 53.3%
Mass of oxygen in sample = 0.533 x 2.8
Mass of oxygen = 1.49 grams
Moles of oxygen = 1.49 / 16
Moles = 0.093
One mole contains 6.02 x 10²³ particles
Thus, the atoms of oxygen:
0.093 x 6.02 x 10²³
= 5.6 x 10²²