Calculate required volume...

Question

Calculate the required volume of Stock 2 (20 mg/mL of ethanol) that you will need to add to each of five volumetric flasks so that when diluted to 100 mL each will yield the desired ethanol concentrations listed above. Be sure to label each flask carefully.

 Standard 1: 100 mL of 0.2 mg/mL ethanol and 2 mg/mL n-propanol in deionised water
 Standard 2: 100 mL of 0.5 mg/mL ethanol and 2 mg/mL n-propanol in deionised water
 Standard 3: 100 mL of 0.8 mg/mL ethanol and 2 mg/mL n-propanol in deionised water
 Standard 4: 100 mL of 1.0 mg/mL ethanol and 2 mg/mL n-propanol in deionised water
 Standard 5: 100 mL of 1.5 mg/mL ethanol and 2 mg/mL n-propanol in deionised water

Answer 1

You can do all of these the same way.  Here are a few examples of using V1C1 = V2C2.  However, I have no idea what the n-propanol is doing there, since the question doesn't mention that at all.

To make 0.2 mg/ml ethanol:  (x ml)(20 mg/ml) = (100 ml)(0.2 mg/ml) and x = 1 ml of the 20 mg/ml

To make 0.5 mg/ml ethanol: (x ml)(20 mg/ml) = (100 ml)(0.5 mg/ml) and x = 2.5 ml of the 20 mg/ml

To make 0.8 mg/ml ethanol: (x ml)(20 mg/ml) = (100 ml)(0.8 mg/ml) and x = 4 ml of the 20 mg/ml

Etc, etc.