Common Registration Assessment, Part 1
Molar solutions and mass per volume
Convert between mol/L and mg/mL, and find the mass for a molar solution, with three original worked questions.
Published 9 October 2026. Independent revision material, not GPhC questions, not for patient care.
A molar concentration counts particles per litre. To move to mass per volume, multiply by the molecular weight. One mol/L of a substance with molecular weight 100 is 100 g/L, which is 100 mg/mL.
The substances are fictional. The method follows the molecular weight approach in the Pharmaceutical Journal medication maths series.
The method
- Write the concentration in mol/L and the molecular weight in g/mol.
- Multiply mol/L by molecular weight to get g/L.
- g/L is the same number as mg/mL.
- For a mass needed, multiply mol/L by the volume in litres, then by molecular weight.
- Check units: g, mg or micrograms, as the question asks.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional compound has molecular weight 100. What is 0.2 mol/L in mg/mL?
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Answer: 20 mg/mL
Working: 0.2 × 100 = 20 g/L, which equals 20 mg/mL.
Question 2
What mass of a fictional salt of molecular weight 58.5 is needed for 250 mL of 0.1 mol/L solution, to 2 decimal places in g?
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Answer: 1.46 g
Working: 0.1 × 0.25 = 0.025 mol. 0.025 × 58.5 = 1.4625, so 1.46 g.
Question 3
A fictional solution contains 5.85 g in 100 mL. Molecular weight is 58.5. What is the concentration in mol/L?
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Answer: 1 mol/L
Working: 5.85 g in 100 mL = 58.5 g/L. 58.5 ÷ 58.5 = 1 mol/L.
Moles as the bridge
Move to moles first, then to mass. Two short steps are safer than one long one.
Where marks are lost
- Forgetting to convert mL to litres.
- Mixing g/L and mg/mL without noticing they are the same number.
- Dividing by molecular weight when you should multiply.
Frequently asked questions
Why is g/L the same as mg/mL?
Both are a thousand-fold change that cancels: 1 g is 1000 mg and 1 L is 1000 mL.
Do I need to know molecular weights?
Use the figure the question supplies.
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
- Osmolarity from component concentrations
- 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.