Common Registration Assessment, Part 1
Millimoles and milliequivalents
Practise converting between mass, millimoles and milliequivalents when the molecular weight and valency are supplied.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
A millimole counts particles. A milliequivalent counts charge, so it is the number of millimoles multiplied by the valency. Questions on these supply the molecular weight, and your task is to apply it in the right order.
The salts and numbers here are fictional. The method follows the molecular weight approach in the Pharmaceutical Journal series: mass divided by molecular weight gives moles.
The method
- Write the mass in mg if you want millimoles directly.
- Divide mass in mg by molecular weight to give mmol.
- Multiply mmol by valency to give mEq.
- For a concentration, divide by the volume in litres.
- Check that you used the valency of the ion, not the salt name.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional monovalent salt has a molecular weight of 80. How many mmol are in 2 g?
Show the answer
Answer: 25 mmol
Working: 2 g = 2000 mg. 2000 ÷ 80 = 25 mmol.
Question 2
A fictional divalent ion is present at 3 mmol. How many mEq is that?
Show the answer
Answer: 6 mEq
Working: mEq = mmol × valency = 3 × 2 = 6 mEq.
Question 3
A fictional solution contains 20 mmol of a monovalent ion in 500 mL. State the concentration in mmol/L.
Show the answer
Answer: 40 mmol/L
Working: 500 mL = 0.5 L. 20 ÷ 0.5 = 40 mmol/L.
Mass in mg gives mmol
If the mass is in mg and the molecular weight is in g per mol, the result is already in mmol. This saves a conversion step.
Where marks are lost
- Dividing grams by molecular weight and calling the result mmol.
- Forgetting valency when asked for mEq.
- Leaving the volume in mL when the unit is per litre.
Frequently asked questions
Is mEq always bigger than mmol?
Not always. For a monovalent ion they are equal. For a divalent ion mEq is double the mmol.
Do I need to memorise molecular weights?
Use the figures the question provides. Do not rely on remembered values.
Sources
- The Pharmaceutical Journal, medication maths: calculations involving molecular weight
- GPhC, 2026 assessment specification and permitted items
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
See also the formula sheet, the eight sample questions and the approved calculator page.