One gram of gold sample will contain 90/100 =0.9g of gold
Number of moles of gold = Mass of gold/ Atomicmass of gold
= 0.9/ 197 = 0.0046
One mole of gold contains NA atoms = 6.022 ×1023
∴ 0.0046 mole of gold will contain = 0.0046 × 6.022 ×1023
= 2.77×1021