Common Registration Assessment, Part 1
Osmolarity from component concentrations
Practise calculating mOsmol/L from g/L, molecular weight and the number of particles, with three original worked questions.
Published 9 October 2026. Independent revision material, not GPhC questions, not for patient care.
Osmolarity counts the particles in a litre. For a substance that does not split, mOsmol/L equals mmol/L. If it splits into two ions, each mmol gives two mOsmol.
The solutions here are fictional. The steps are mass to moles, then moles to particles, then add components.
The method
- Convert g/L to mmol/L: divide by molecular weight, then multiply by 1000.
- Multiply by the number of particles each unit gives.
- Add the contributions of all components.
- Round only at the end.
- Check the units are mOsmol/L.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional salt has molecular weight 58.5 and splits into two ions. It is present at 5.85 g/L. Find the osmolarity in mOsmol/L.
Show the answer
Answer: 200 mOsmol/L
Working: 5.85 ÷ 58.5 = 0.1 mol/L = 100 mmol/L. × 2 particles = 200 mOsmol/L.
Question 2
A fictional sugar of molecular weight 180 does not split. It is present at 50 g/L. Find the osmolarity, to the nearest whole number.
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Answer: 278 mOsmol/L
Working: 50 ÷ 180 = 0.2778 mol/L = 277.8 mmol/L, so 278 mOsmol/L.
Question 3
A fictional solution has the salt from the first question at 5.85 g/L and the sugar from the second at 18 g/L. What is the total osmolarity?
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Answer: 300 mOsmol/L
Working: Salt = 200. Sugar = 18 ÷ 180 × 1000 = 100. Total = 300 mOsmol/L.
Particles per unit
Write how many particles each unit gives next to the molecular weight before you start.
Where marks are lost
- Forgetting to double for a salt that gives two ions.
- Mixing mmol and mol.
- Adding g/L values instead of osmoles.
Frequently asked questions
Is osmolarity the same as osmolality?
No. Osmolarity is per litre of solution and osmolality is per kg of solvent. Use the one the question names.
Why add components?
Each dissolved particle counts, whichever substance it comes from.
Sources
- The Pharmaceutical Journal, medication maths: calculations involving molecular weight
- The Pharmaceutical Journal, preparing for the GPhC pharmaceutical calculations assessment
- 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
- Millimoles and milliequivalents
- 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
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
See also the formula sheet, the eight sample questions and the approved calculator page.