Common Registration Assessment, Part 1
Moles and millimoles calculations
Three original questions linking mass, molecular weight and concentration in mmol.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Moles and millimoles connect a mass to a number of particles through the molecular weight. A mole of a substance weighs its molecular weight in grams, so a millimole weighs that figure in milligrams.
That means mmol = mg ÷ molecular weight. Concentrations follow: 1 mol/L is 1 mmol/mL.
The method
- Convert the mass to mg if needed.
- Divide by molecular weight for mmol, or multiply for mass.
- For concentration, 1 mol/L = 1 mmol/mL.
- State the unit in the answer.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
How many mmol of potassium chloride (molecular weight 74.5) are in 1.5 g?
Show the answer
Answer: 20.1 mmol
Working: 1.5 g = 1,500 mg. 1,500 ÷ 74.5 = 20.1 mmol.
Question 2
How many mg of sodium chloride (molecular weight 58.5) contain 25 mmol?
Show the answer
Answer: 1,462.5 mg
Working: 25 × 58.5 = 1,462.5 mg, which is 1.4625 g.
Question 3
A 150 mL volume of a 0.2 mol/L solution is used. How many mmol does it contain?
Show the answer
Answer: 30 mmol
Working: 0.2 mol/L = 0.2 mmol/mL. 0.2 × 150 = 30 mmol.
Molar mass in one line
The Pharmaceutical Journal's molecular weight article explains that molar mass is the sum of the atomic masses of all atoms in the molecule, in grams per mole, and that this value is what converts between moles and grams. Where a question gives you the molecular weight, use it as stated.
Where marks are lost
- Mixing grams with the milligram form of the formula.
- Confusing mol/L with mmol/L.
- Using atomic weight where the question gives molecular weight.
Frequently asked questions
What is the difference between mol and mmol?
1 mol is 1,000 mmol. The numbers differ by a factor of 1,000, so check which the question wants.
What if no molecular weight is given?
Questions normally supply it. If it is not given, the calculation is not expected to need it.
Sources
- The Pharmaceutical Journal, medication maths: calculations involving molecular weight (24 March 2025)
- GPhC, Common Registration Assessment specification and permitted items for 2026
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
- Displacement volume calculations for reconstitution
See also the formula sheet, the eight sample questions and the approved calculator page.