You said the compound was Cadmium Selenide, which as a molecular weight of 191.4 g/mol.
You said it says 69.4 nanomoles / 1mL
69.4 nanomoles = 69.4 (10-9
* 191.4 g = 13.2 (10-6
) g = 13.2 μg
The molar extinction coefficient is the older term for the molar absorptivity coefficient. It is a constant for a specific substance in a specific solvent. It relates the absorbance of the compound to its concentration and path length in the Beer-Lambert law.
Your material is not juse Cadmium Selenide though as you previously said. Based on the "Core Shell Evidot" comment, it is actually a more complicated mixture with Zinc Sulfide and such, not just the CdSe. But who knows what proportions the ZnS and CdSe are in, so the molecular weight can not be added up from constitute parts.
The best you can do is take the bottles word.
You say the bottle says 69.4 nmol/mL and 10mg/mL, which means there are 10mg/69.4 nmol10 (10-3) g
= 144,092 g/mol
But your quantum dots also probably have amines in them as well to "cap" them.
But back to your first question:
My question is to calculate the number of molecules per milliliter of a solution?
You are told there is 69.4 nmol's of stuff in 1 mL. Therefor, multiplying it by avagrados number gives you how many molecules of stuff in 1mL.
This is a very poor question and answer though.