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PhAST Track Mathematics Module: Relevance to Pharmacy

Moles in Pharmacy - Electrolytes

Why is this relevant to pharmacy, you ask? Well, there are a few reasons.

Firstly, millimoles are the most common way to express concentration of electrolytes. If you are mixing TPN for a patient, you will need to be able to use these calculations to determine appropriate quantities of additives.

WATCH OUT!!!

Pharmacy students really seem to struggle with mole calculations. Here are the two most common errors:

  1. Not keeping units consistent with the formula - there are two potential sources of error here:
      • In pharmacy, we tend to state drug quantities in milligrams, but in the formula we need to use GRAMS
        • Again, in pharmacy, millimoles are more commonly used, but the formula uses MOLES
      1. Forgetting to adjust the number of moles depending on the chemical formula (e.g., Lithium carbonate, Li2CO3 contains TWO moles of lithium for every ONE mole of lithium carbonate) 

      Moles in Pharmacy - Multiple drug forms

      Also, drugs are often formulated in their salt form (usually to improve their solubility). A problem occurs when multiple salt forms (or even just a salt form and the pure drug base) are available for use, as the amount of base drug in each salt form will differ.

      For example, in Australia, morphine is available in three salt forms:

      • Morphine sulfate
      • Morphine hydrochloride
      • Morphine tartrate

      We may need to calculate dose-equivalence between salt forms based on the amount of pure drug…

      Moles in pharmacy - Converting between drug base and drug salts

      Usually, drugs are scheduled (classified according to their relative safety) according to content of pure drug (also known as "drug base") .

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