Calculate the mass

Question

Calculate the mass in mg of kanamycin required to make 25 mL of a 12 mg/mL solution of kanamycin.

Answer 1

25 ml of a 12 mg/ml soln would require 25x12 = 300 mg per 25 mls. Note that if you keep the unit labels in the equation, that the ml units cancel (mg/ml x ml = mg).

There is an alternative way of doing this as well. Old lab rats, (folks like me) often use the concept of thousand fold conversion. Which means that you can do equivalent conversions as you move the units a thousand fold in any direction. Thus a 12 mg/ml soln is equivalent to a 12g/L soln. This concept is particularly useful if you don't have a scale/balance that measures milligrams accurately. Applying this further we can make a 1.2 g/100 ml soln, which is the same as the 12mg/ml soln.