(i) 1 mole of oxygen contains 6.022 x 1023 atoms
∴ 6.022 x 1023 atoms of oxygen = 1 mol
1 atom of oxygen = 1 / 6.022 × 1023 mol
∴ 3.01 1 x 1023 atoms of oxygen = (1 × 3.011 × 1023) / (6.022 × 1023) mol
= 0.5 mol
(ii) Atomic mass of Ca = 40 u
40g of calcium = 1 mol
60g of calcium = 60/40 mol
= 1.5 mol