An atom of zinc has a mass of 65.39 amu. One of its isotopes, image038, has an 18.75% abundance. Since 1 amu = 1.6606 x 10-24 g, how many of atoms of this zinc isotope would be present in a 1.00 g sample of zinc?

Question

cuanchi · Answer

I assume that the atom of zinc mass of 65.39 amu is the average mass of the Zn atom.
 I think you will need to know the mass of the isotope to calculate the number of atoms of the isotope. 
I can only tell you that with the 18.75% abundance, in one gram of Zn you will have 0.1875 g of the isotope, but there will be other 81.25% of other isotope(s) that can be heavier or lighter than this one making the average mass 65.39 amu. 
If 65.39 is the mass of the isotope I can divide 0.1875 g  by the mass in g of an average atom and get 1.7 x 10^21 atoms.
 If the isotope that we are looking for is heavier than 65.39 amu we will have less atoms than 1.7 x 10^21