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?

One gram of gold sample will contain $\frac{90}{100}=0.9$ g of gold 
Number of moles of gold $=\frac{\mathit{Mass\; of\; gold}}{\mathit{Atomic\; mass\; of\; gold}}=\frac{0.9}{197}$
                                                                           $\Rightarrow 0.0046$
N_A atoms are present in one mole of gold $=6.022\times {10}^{23}$
$\therefore$ 0.0046 mole of gold will contain $=0.0046\times 6.022\times {10}^{23}$
                                                        $\Rightarrow 2.77\times {10}^{21}$ atoms
 

The sample containing $90\%$ of gold and the rest copper have 2.77 $\times$ 10²¹ atoms of gold are present in one gram of this sample of gold.