A question may ask you to prepare a solution that equals a certain amount. Like how many mg/ml. For instance, if you had a vial that had 10 mg and it wanted you to reconstitute it, the best thing to do is to make it the mg = ml if you wanted a certain ratio. You would dilute 10 mg with 10 ml NS to get a 1:1 ratio or 1 mg/ml. Or, 5 ml NS in a 10mg vial to get a 2:1 ratio or 2mg/ml, instead of 1 mg/ml. But as the PP said, it depends on the drug. Some meds need a specific volume while others can be freely diluted for a more accurate measurement for infusion. If you were told to inject 25 mg from a vial that contained 100 mg powder, you would dilute with 4 ml of NS to equal 25 mg/ml (100mg/4ml with reconstitution) and inject 1 ml (25 mg). Once you have the final mg/ml, you can calculate the volume needed for injection. The best thing in this case is to divide the vial dose from the needed dose (100/25 = 4 = 4 ml of dilutant needed).
100 ml NS in 100 mg vial = 1 mg/ml 1:1
50 ml NS in 100 mg vial = 2 mg/ml 2:1
25 ml NS in 100 mg vial = 4 mg/ml 4:1
5 ml NS in 100 mg vial = 20 mg/ml 20:1
So, if you had a 2000 mg vial and you had to inject 750 mg, you would reconstitute with 16 ml of NS to equal 125 mg/ml (2000/16 = 125) and inject 6 ml. (6 X 125 = 750). Or, you could dilute with 8 ml NS and inject 3 ml (8 ml of dilutant gives you concentration of 250mg/ml, and 3 ml X 250 mg = 750 mg. Or, dilute with 4 ml NS and inject 1.5, and so on.
The problem with the above equations is exams usually give you the dilutant volume from the start. They will tell you that you have so many mg, which is diluted with so many ml, and you have to figure out how much to infuse or how fast. If they want to ask you the best way to safely dilute it to get an accurate measurement, then you could use the above equations. Like, you wouldn't dilute a 5 ml vial with 3.6 ml NS b/c that gives you a concentration of 1.39 mg/ml, but a question might throw that volume at you in order to just test you. Or, they would ask have many ml would you inject for 2.78 mg and you would say 2 ml (5 mg /3.6 ml = 1.38 mg/ml. 2.78mg/ 1.38 = 2 (ml).
The other problem is questions aren't going to ask about a dilutant in a vial. It's going to mixed in a bag. So, most of time, the meds are diluted in 50ml bags, or 250 ml bags or 1000. So, you have to figure out the concentration of the bag. How many mg in this 250 bag and how fast to infuse it.
You can be asked to reconstitute a 100 mg vial and dilute it in a 1000 ml NS. You just divide it and you get the mg/ml and the ratio (10:1). Then, you can calculate how many mg/hr you need or how many ml/hr for the answer.
A question may specifically ask if you can recall the proper dilution for a medication (although this is probably nonexistent. I don't ever remember getting tested on the volume of a dilutant). But, vancomycin....1 gm of vancomycin is diluted in 250 ml of NS. That's normally what is done. But, those volumes can vary for many different reasons, so I doubt they would want to know that unless they specifically mentioned it in class.
But yes, you have to find out how many mg is available and then find out how many mg is desired. The main thing is mg/ml after mixing in the bag, or (whatever measurement is asked like micrograms/ml). That is termed "final concentration." The final concentration will lead you toward the goal rate. And just to be clear, it doesn't matter if you reconstitute with a specific amount of NS and then mix it in a bag. The bag is basically the final concentration. You use the NS from the bag. If you use a NS vial of 10 ml and inject that into a bag, then the final amount is 110ml, not 100 ml. Reconstituting just a vial only matters if you are injecting straight from the vial. That's when the 5mg/ml vial is diluted with 5 ml NS to = 1 mg/ml or 5 mg/5 ml or 1:1.
I hope I answered what you were asking