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?

Explanation

Let, the mass of the sample is 100g.
Mass of gold =90g
and mass of copper `=(100-90)=10g`
`therefore` 100g of sample has gold =90g
`therefore` 1g of this sample has gold `=90/100=0.9g`
Atomic mass of gold (Au) =197g
`therefore` 197 g of gold have number of atoms =`6.022 xx 10^(23)`
`0.9g` of gold will have number of atoms `=(6.022 xx 10^(23) xx 0.9)/(197) = 2.75 xx 10^(21)`