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    Objectives: Upon completion of the chapter and exercises, the student pharmacist will be able to
  1. Interpret a prescription or medication order.

  2. Convert between the metric and common systems of measurement.

  3. Calculate an appropriate dose.

  4. Prepare, concentrate, or dilute compounded medications accurately.

  5. Interpret osmolarity, isotonicity, and milliequivalents.

  6. Prepare isotonic solutions.

  7. Reconstitute dry powders to appropriate concentration.

  8. Utilize the aliquot method for solids and liquids.

Patient Encounter

In the accident, patient X, suffered a deep abrasion on his leg and may eventually need a skin graft. Signs of infection are present and the decision has been made to irrigate the wound daily with modified Dakin's (diluted sodium hypochlorite) solution and use it on the wound dressing. You consult with the pharmacist in charge to confirm that the pharmacy has all of the ingredients. They are as follows:

  • Clorox bleach (standard 5.25% strength, not “ultra”)

  • Sodium bicarbonate (1/2 tsp per quart)

  • Sterile water

  • The hospital order merely says the following:

  • Modified Dakin's solution 1/4%, gal #1

  • Use daily for wound cleansing and dressing on right leg

Questions to follow up in this case include:

  1. How many milliliters of Clorox would be needed to make a gallon of this irrigation?

  2. Who do you need to go to in the pharmacy to see that this order is correctly processed?

  3. What are the procedures in the pharmacy for double check on the calculations?

  4. Why did you choose this over an antibiotic?

Solution follows:

(5.25%) (X)=(0.25)(3785 mL)X=[(0.25%)(3785 mL)]/5.25%=180 mL


The profession of pharmacy is one in which mathematics is used extensively. With the exception of some pharmacokinetic expressions, most of the calculations you will be expected to perform will be simple arithmetic. That being said, the importance of not making errors in your arithmetic cannot be overemphasized. An error in a calculation by a pharmacist could easily be the difference between life and death. You should develop several methods to check your work as you go. Estimate the final answer before beginning the work. Anything quite different from your approximation should cause you to reexamine your work. As you progress further into your pharmacy education and your career, you will become more comfortable with estimating what these answers should be in a given situation and for a given patient, such that you can identify an unreasonable solution relatively easily.

In order for this chapter to be brief, basic math is not reviewed. There are a multitude of reliable resources for such material. You are expected to be able to perform basic arithmetic (addition, subtraction, division, and multiplication) and to do so not only with whole numbers or integers but with fractions as well. Last, you are expected to be able to ...

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