We are asked to determine the number of atoms in 4.5 moles of hydrogen. To do that we will Avogadro's number:
[tex]1mol=6.023\times10^{23}atoms[/tex]Using the conversion we get:
[tex]4.5mol\times\frac{6.023\times10^{23}atoms}{1mol}=27.1\times10^{23}atoms[/tex]