Calculations
mmol/mEq Electrolyte Calculations for OPRA: Worked Examples and MCQs
Electrolyte results and replacement products are reported in millimoles or milliequivalents, not milligrams — and converting correctly between the three depends on a single relationship (molar mass and valence) that's easy to apply once it clicks and easy to get backwards under time pressure. This OPRA pharmacy calculations guide covers that relationship, worked potassium, sodium and magnesium examples, and the mistakes that cost marks.
Why this topic matters
Electrolyte results on a pathology report, and the doses on electrolyte replacement products, are given in mmol or mEq rather than mg — so a candidate who can only think in milligrams is stuck before the clinical part of the question even starts. This conversion also underpins reading a report correctly: mistaking mmol/L for mg/dL, or vice versa, changes whether a result looks normal or dangerously abnormal.
Learning objectives
- Convert a mass (mg or g) of an electrolyte salt to millimoles (mmol) using its molar mass
- Convert between millimoles and milliequivalents (mEq) using an ion's valence
- Apply these conversions to a worked potassium, sodium or magnesium replacement calculation
- Recognise the electrolyte salts most likely to appear in an OPRA calculation scenario, and the valence trap OPRA likes to test
Core concepts
Millimoles: mass divided by molar mass
A mole represents a fixed amount of a substance (a set number of atoms, ions or molecules) — a millimole (mmol) is one-thousandth of a mole. To convert a mass of a substance to millimoles: mmol = mass (mg) ÷ molar mass (mg/mmol), where molar mass is the same number as the substance's molecular weight in g/mol, just expressed as mg/mmol. For example, potassium chloride (KCl) has a molar mass of approximately 74.5 g/mol (74.5 mg/mmol), so 745 mg of KCl contains 745 ÷ 74.5 = 10 mmol of KCl — and because each KCl molecule provides exactly one potassium ion, this is also 10 mmol of K⁺.
The mmol ↔ mEq formula, at a glance
The full relationship in one line: mEq = mmol × valence.
- Monovalent ions — K⁺, Na⁺, Cl⁻ (valence 1): 1 mmol = 1 mEq
- Divalent ions — Ca²⁺, Mg²⁺ (valence 2): 1 mmol = 2 mEq
Milliequivalents: millimoles adjusted for valence
A milliequivalent (mEq) accounts for the electrical charge an ion carries, not just its quantity. For a monovalent ion (charge of ±1) — such as potassium (K⁺), sodium (Na⁺) or chloride (Cl⁻) — 1 mmol = 1 mEq, so the two units are interchangeable. For a divalent ion (charge of ±2) — such as calcium (Ca²⁺) or magnesium (Mg²⁺) — 1 mmol = 2 mEq, because each mole of the ion carries twice the charge-equivalents.
Why this distinction actually matters clinically
Potassium and sodium are monovalent, so in Australian practice their results and replacement doses are conventionally reported in mmol, and mmol and mEq can be used interchangeably without a conversion step. Calcium and magnesium are divalent, so a result or dose given in mEq is not numerically the same as one given in mmol for these ions — a candidate who assumes 1 mmol = 1 mEq across the board will misread or miscalculate a calcium or magnesium figure specifically.
Clinical application
Worked example — potassium replacement from mass
A potassium chloride ampoule contains 1.5 g of KCl. How many mmol of K⁺ (potassium ions) does it provide? Molar mass of KCl ≈ 74.5 mg/mmol. mmol of KCl = 1500 mg ÷ 74.5 mg/mmol ≈ 20.1 mmol. Because each KCl molecule contains exactly one potassium ion, mmol of KCl equals mmol of K⁺ — so this is ≈ 20 mmol of potassium. And because potassium is monovalent, that's also ≈ 20 mEq — the same figure again, no further conversion needed.
Worked example — converting a calcium dose between units
A product provides 10 mEq of elemental calcium per dose. How many mmol is this? Because calcium is divalent (mEq = mmol × 2), mmol = mEq ÷ valence = 10 ÷ 2 = 5 mmol. Reading this the other way — treating the 10 mEq figure as if it were 10 mmol — would overstate the elemental calcium content by a factor of two.
Worked example — mass of salt needed for a target mmol of ion
A preparation requires 40 mmol of sodium. How many grams of sodium chloride (NaCl) are required? NaCl provides exactly 1 mmol of sodium per 1 mmol of NaCl, so 40 mmol of sodium needs 40 mmol of NaCl. Molar mass of NaCl ≈ 58.5 mg/mmol, so mass = 40 × 58.5 = 2340 mg ≈ 2.34 g of NaCl.
The classic OPRA trap — forgetting valence on the way from mmol to mEq
A magnesium sulfate preparation contains 20 mmol of magnesium ions. How many mEq of magnesium does it contain? A candidate who assumes mmol and mEq are interchangeable (true for potassium or sodium) answers 20 mEq — and gets it wrong. Magnesium is divalent, so mEq = mmol × valence = 20 × 2 = 40 mEq. This valence step is the single most commonly tested trap in this calculation type, precisely because it's invisible unless you stop and check which ion you're working with.
Common mistakes
- Applying 1 mmol = 1 mEq to a divalent ion (calcium, magnesium) — this only holds for monovalent ions such as potassium, sodium and chloride.
- Confusing the molar mass of the whole salt (e.g. KCl, ≈74.5 mg/mmol) with the molar mass or valence of the ion of clinical interest (e.g. K⁺ alone) when a question asks specifically about elemental potassium content.
- Reading a lab result in the wrong unit — mistaking mmol/L for mg/dL (or vice versa) changes whether a result looks normal, low or dangerously high.
- Rounding the molar mass too early in a multi-step calculation, compounding a small error across several conversions.
Exam tips
- • If a stem names calcium or magnesium and gives a dose in mEq, treat that as a signal the mmol figure is half (calcium, magnesium) the mEq figure — the valence-of-2 conversion is a favourite way to test whether this distinction is genuinely understood.
- • Write out the formula with units before substituting numbers (mmol = mg ÷ [mg/mmol]) — this makes it obvious if you've accidentally divided the wrong way round under time pressure.
- • When a question gives a salt's mass and asks for the ion content in mmol, check whether it's asking about the whole salt or just one ion within it (e.g. calcium gluconate vs. elemental calcium) — the molar mass used differs.
Memory tricks
- • "One for one, unless it's two" — potassium, sodium and chloride convert mmol to mEq one-for-one; calcium and magnesium (divalent) need the valence-of-2 step.
- • Before answering any mmol/mEq question, name the ion out loud and its charge — K⁺/Na⁺/Cl⁻ (×1) or Ca²⁺/Mg²⁺ (×2) — then apply mEq = mmol × valence. This one habit catches the magnesium/calcium trap almost every time.
Clinical pearls
- 💡 Australian electrolyte reporting and replacement-product labelling is conventionally in mmol, not mEq — mEq appears more often in older references or non-Australian sources, so recognising both and converting confidently between them matters even though mmol is what you'll see day to day.
- 💡 Elemental electrolyte content differs by salt form — for example, calcium gluconate and calcium chloride provide different amounts of elemental calcium per gram of salt, because the non-calcium portion of the molecule takes up a different share of the total molar mass.
Tables
Electrolytes worth knowing for OPRA — quick reference
| Electrolyte | Charge | Conversion |
|---|---|---|
| Potassium (K⁺) | +1 | mmol = mEq |
| Sodium (Na⁺) | +1 | mmol = mEq |
| Chloride (Cl⁻) | −1 | mmol = mEq |
| Calcium (Ca²⁺) | +2 | 1 mmol = 2 mEq |
| Magnesium (Mg²⁺) | +2 | 1 mmol = 2 mEq |
Common electrolytes: valence and approximate molar mass
| Ion / salt | Valence | mmol : mEq ratio | Approx. molar mass |
|---|---|---|---|
| Potassium (K⁺) / KCl | 1 | 1 : 1 | KCl ≈ 74.5 mg/mmol |
| Sodium (Na⁺) / NaCl | 1 | 1 : 1 | NaCl ≈ 58.5 mg/mmol |
| Chloride (Cl⁻) | 1 | 1 : 1 | — |
| Calcium (Ca²⁺) | 2 | 1 mmol : 2 mEq | Varies by salt (gluconate vs. chloride) |
| Magnesium (Mg²⁺) | 2 | 1 mmol : 2 mEq | Varies by salt (sulfate vs. chloride) |
Practice MCQs (100% original)
1. A potassium chloride ampoule contains 745 mg of KCl (molar mass ≈ 74.5 mg/mmol). How many mmol of potassium does this provide?
2. A calcium product label states it provides 10 mEq of elemental calcium. Approximately how many mmol of calcium is this?
3. A patient requires 30 mmol of sodium chloride added to an infusion bag. Sodium chloride has a molar mass of approximately 58.5 mg/mmol. What mass of NaCl should be added?
4. A magnesium sulfate preparation contains 20 mmol of magnesium ions. How many mEq of magnesium does it contain?
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Start freeFrequently asked questions
Is 1 mmol always equal to 1 mEq?
Only for monovalent ions — potassium, sodium and chloride are the most commonly tested examples, where mmol and mEq are numerically interchangeable. For divalent ions such as calcium and magnesium, 1 mmol = 2 mEq, so the two units are not the same number for those ions.
Why do Australian lab results usually use mmol rather than mEq?
mmol is the standard SI-derived unit used in Australian pathology reporting and product labelling. mEq appears more often in older or non-Australian references, so it's still worth being able to convert confidently between the two, even though mmol is what you'll encounter most often in practice.
Does the molar mass of a salt (e.g. KCl) give the same mmol figure as the molar mass of just the ion (e.g. K⁺)?
No — a question about the elemental ion content of a salt uses the ion's own contribution to the molar mass, not the whole salt's molar mass, if it's specifically asking about a different quantity (such as one ion within a multi-ion compound). Read carefully whether a question is asking about the salt as dispensed or the elemental ion content.
Do the practice MCQs below come with explanations?
Yes — every option, not just the correct one, has its own explanation. For example, on the potassium question below: the correct answer (10 mmol) is explained as 745 mg ÷ 74.5 mg/mmol = 10 mmol, while each wrong option is explained too — 7.45 mmol comes from dividing by 100 instead of the molar mass, 74.5 mmol is the molar mass itself rather than the result of dividing by it, and 20 mmol doesn't follow from the given figures at all. Click any option in the MCQs below to see this for every question.
Official references
- Australian Medicines Handbook ↗ — Product-specific electrolyte content and replacement dosing
- Therapeutic Guidelines Australia ↗ — Electrolyte replacement and monitoring guidance