[enahs]

Because you do not know the proportions the two are mixed. They are not forming a new compound with a chemical reaction in this case, you have the ZnS as a physical shell around the CdSe.
There could be 3 ZnS for every one CdSe, or 500. Plus there is most likely an amine in there as well, again you do not know the proportion.

[fudawala]

Now I understand, hey thanks a lot.

But, how would I solve for the number of particles per millimeter if the molecular weight is no use. Because you said that the ZnS and the CdSe are inter mixed with each other.

[enahs]

You can only trust what was given to you as 69 nano moles of stuff per 1 ml.

If you have 1 ml of the stuff, and 69 nano moles in it, you then have 69 nano moles * avagrados number of molecules

Avagrados # is
# of "stuff"
  1 mol

And in this case, the "stuff" is molecules. Avagrados number is just like a dozen, just a number. You can have a dozen eggs or a dozen waffles.

But remember Avagrados number is in mols, so you must convert nano mols to mols first.

[fudawala]

So Once I know Avogrado's Number which is 1 mole = 6.02 X 10^23 atoms right and when I convert 1 nanomole to moles which is:

1 nanomole = 10^-9 mole right. This will tell me the number of particles per milliliter right. Thanks again.

[fudawala]

Can I ask you what happens to the density of these two compunds. Should I consider it in his problem?

[Hapto]

Are you sure that you read the label correctly?  Quantum dots have both the wavelength of absorption (useful for determination of size of the dots) and the weight percent of the suspended particles (useful for concentration of the dots) on the label.

Are you sure that the label didn't say 694 nm (nanometers! ... not nanomolar) and 10 mg/ml?