A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?
In the given sample, gold (Au) is 90%.
Thus, One gram of gold sample will contain = 
The molar weight of gold = 197g/mole
The number of moles of gold (Au) = 
We know, 1mole = 6.022 x 1023
∴ 0.0046 mole of gold will contain = 0.0046 × 6.022 ×1023
= 2.77 ×1021 atoms of gold particle
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