The volume occupied by 500mg of ceftriaxone is 0.2mls, we want to reconstitute the ceftriaxone to 500mg/4mls, how much diluent (sterile water or NS) would we add to achieve this concentration?

Question

The volume occupied by 500mg of ceftriaxone is 0.2mls, we want to reconstitute the ceftriaxone to 500mg/4mls, how much diluent (sterile water or NS) would we add to achieve this concentration?

my answer was 20 mL

Answer 1

If you add 20 mls of diluent to something that occupies 0.2 mls, wouldn't the final volume be 20.2 mls? So, if you want the final volume to be 4 mls, and the material (drug) occupies 0.2 mls, you'd want to add 4 mls - 0.2 mls = 3.8 mls. And since the mass of drug that occupies 0.2 mls is 500 mg, you would now have a final concentration of 500 mg/4 mls. Does that make sense?