Calculate the number of moles for 20g of Helium atoms, if the molar mass of helium is 4g.

 The molar mass of an atom is the atomic mass of that atom expressed in grams. It is also known as gram atomic mass.

$\mathrm{Given}: \mathrm{Mass} \mathrm{of} \mathrm{the} \mathrm{given} \mathrm{substance} \left(\mathrm{m}\right)= 20 \mathrm{g}$ 

             $\mathrm{Molar} \mathrm{mass} \mathrm{of} \mathrm{Helium} \mathrm{atom} \left(\mathrm{M}\right)= 4\mathrm{g}$ 

$\mathrm{Solution}:$

$\mathrm{Number} \mathrm{of} \mathrm{moles} = \frac{\mathrm{Mass} \mathrm{of} \mathrm{substance} \left(\mathrm{m}\right)}{\mathrm{Molar} \mathrm{mass} \mathrm{of} \mathrm{given} \mathrm{substance} \left(\mathrm{M}\right)}$

                                   $= \frac{20\mathrm{g}}{4\mathrm{g}}$

                                   $= 5 \mathrm{moles}$

$\mathrm{The} \mathrm{number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{helium} \mathrm{atoms} \mathrm{in} 20\mathrm{g} \mathrm{of} \mathrm{sample} \mathrm{is} 5.$