Here's how to calculate the mass of ethanol molecules in milligrams:

1. Convert Molecules to Moles

* Avogadro's number: 6.022 x 10^23 molecules/mole

* Calculation: (5.1 x 10^20 molecules) / (6.022 x 10^23 molecules/mole) = 8.47 x 10^-4 moles

2. Calculate Mass in Grams

* Molar mass of ethanol (C2H5OH): 46.07 g/mol

* Calculation: (8.47 x 10^-4 moles) * (46.07 g/mol) = 0.039 g

3. Convert Grams to Milligrams

* Conversion factor: 1 g = 1000 mg

* Calculation: (0.039 g) * (1000 mg/g) = 39 mg

Therefore, 5.1 x 10^20 molecules of ethanol have a mass of approximately 39 mg.