Moles & millimoles

Convert between a mass of substance and the number of moles or millimoles it represents.

What is this for?

A mole is the molecular weight of a substance expressed in grams; a millimole is that weight in milligrams. Molar quantities underpin osmolarity, milliequivalent, and electrolyte-replacement calculations, and phosphate products are labelled in millimoles rather than milliequivalents.

How to use it

Choose the direction and the scale — milli for the mg/mmol pair, base for the g/mol pair.
Enter the amount and the molecular weight of the exact salt form you are using.
The result panel shows the answer on both scales so you do not have to convert separately.

Worked example

How many millimoles of sodium chloride are in 500 mg?

Answer: 500 ÷ 58.5 = 8.55 mmol. Since sodium chloride is monovalent, this is also 8.55 mEq of sodium.

Clinical pearls & pitfalls

Use the molecular weight of the salt you actually have. Calcium chloride dihydrate (147.0) and anhydrous calcium chloride (111.0) differ by a third — using the wrong one is a substantial dosing error.
Phosphate is dosed in millimoles of phosphorus precisely because its charge, and therefore its milliequivalent value, changes with pH. Never convert a phosphate order to mEq.
The mole is about particle number, the equivalent about charge. They coincide only for monovalent ions.

Assumptions & limitations

Requires an accurate molecular weight for the specific hydrate and salt form in use.
Does not account for the purity or potency designation of a raw material. A chemical assaying at 98% requires a proportionally larger weight.

References

International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology (Gold Book), definition of the mole.
United States Pharmacopeia–National Formulary monographs (molecular weights and hydrate forms).