Number of moles = given mass/ molar mass
Given mass of calcium = 0.8 g
Atomic mass of calcium = 40g /mol
Number of moles = 0.8/40
= 0.02 mol
ā1 mol = 6.02210 23 molecules
0.02 mol = 0.12044ā10 23 molecules
Hence, 08g of calcium has ā0.12044ā10 23 molecules.