So first we must find how many moles this amount of atoms is. We know that Avogadro's number is the amount of atoms per mole:
[tex] \frac{1mole}{6.022x10^{23} atoms} *[1.3*10^{21}atoms]=2.16*10^{-3}mol [/tex]
So then we look to the periodic table to find the molar weight of sodium (Na).
Sodium's molar weight is 22.99g/mol. So let's solve for the amount of grams:
[tex] \frac{22.99g}{1mol}*[2.16*10^{-3}mol]=4.97*10^{-2}g [/tex]
So now we know that the given amount of atoms weighs [tex] 4.97*10^{-2}g [/tex].
