Question 39
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?

Let the mass of the sample is 100g.
Mass of gold = 90g
Mass of copper = (100-90) = 10g
One gram of gold sample will contain $\frac{90}{100}=0.9gofgold$
Number of moles of gold $=\frac{Massofgold}{Atomicmassofgold}$
$=\frac{0.9}{197}=0.0046$
One mole of gold contains $N_A$ atoms $=6.022\times {10}^{23}$
$\therefore$ 0.0046 mole of gold will contain $=0.0046\times 6.022\times {10}^{23}$
$=2.77\times {10}^{21}$