Module 6 of 14 · 12 min
Reconstitution & powder volume
Antibiotic suspensions, injectable vials and biologics arrive as powders. The powder itself takes up space, so the diluent volume is never the final volume — the exam checks whether you account for it, especially when a non-standard concentration is ordered.
🧠 Powder takes up room: Final = Diluent + Powder (F = D + P).
When a powder dissolves it displaces volume. If the label says 'add 9.6 mL to make 100 mg/mL' for a 1 g vial, the final volume is $1000\,\text{mg} \div 100\,\text{mg/mL} = 10\,\text{mL}$, so the powder occupies $10 - 9.6 = 0.4\,\text{mL}$.
Powder volume is a property of the vial or bottle — it does not change with how much diluent you add. Find it once from the label's directions, then reuse it to make any other concentration.
For a new concentration: the final volume needed is (total drug ÷ wanted concentration); subtract the powder volume to get the diluent to add. The same logic works with units (e.g. penicillin G in units/mL) as with mg.
Once reconstituted, the dose volume is dose ÷ concentration — and the concentration is total drug ÷ FINAL volume, not ÷ diluent volume.
🧭 The method
- 1From the label: final volume = total drug ÷ labelled concentration.
- 2Powder volume = final volume − labelled diluent volume.
- 3For the wanted concentration: new final volume = total drug ÷ wanted concentration.
- 4Diluent to add = new final volume − powder volume.
- 5Dose volume = dose ÷ concentration actually produced.
Before each reveal, predict the next step — then check.
A ceftriaxone 1 g vial: the label says add 9.6 mL of sterile water to obtain 100 mg/mL. You need 250 mg/mL for an IM injection. How much diluent, and what volume is a 750 mg dose?
- 1
Final volume made by following the label.
$$\frac{1000\,\cancel{\text{mg}}}{100\,\cancel{\text{mg}}/\text{mL}} = 10\,\text{mL}$$ Powder volume = final volume − diluent added.
$$10\,\text{mL} - 9.6\,\text{mL} = 0.4\,\text{mL}$$Final volume needed for 250 mg/mL.
$$\frac{1000\,\text{mg}}{250\,\text{mg/mL}} = 4\,\text{mL}$$Diluent to add = new final volume − powder volume.
$$4\,\text{mL} - 0.4\,\text{mL} = 3.6\,\text{mL}$$Dose volume.
$$750\,\cancel{\text{mg}}\times\frac{1\,\text{mL}}{250\,\cancel{\text{mg}}} = 3\,\text{mL}$$
Same method — the last steps are yours. Work on paper, then reveal.
A penicillin G potassium 5,000,000-unit vial gives 250,000 units/mL when 18.2 mL of diluent is added. How much diluent gives 500,000 units/mL, and what volume is a 1,200,000-unit dose?
- 1
Final volume per the label.
$$\frac{5{,}000{,}000\,\text{units}}{250{,}000\,\text{units/mL}} = 20\,\text{mL}$$ - 2
Powder volume.
$$20 - 18.2 = 1.8\,\text{mL}$$ - 3
Your turn
Now you: final volume for 500,000 units/mL, then subtract the powder volume.
Work it out on paper, then reveal.
$$\frac{5{,}000{,}000}{500{,}000} = 10\,\text{mL};\quad 10 - 1.8 = 8.2\,\text{mL diluent}$$ - 4
Your turn
Now you: volume for the 1,200,000-unit dose.
Work it out on paper, then reveal.
$$\frac{1{,}200{,}000\,\text{units}}{500{,}000\,\text{units/mL}} = 2.4\,\text{mL}$$
Type your number. Your first try decides when this module comes back for review.
An amoxicillin bottle says: add 78 mL of water to make 100 mL of 250 mg/5 mL suspension. What is the powder volume?
Final volume − water added.
= 22 mL
A 2 g vial has a powder volume of 1.4 mL. You add 8.6 mL of diluent. What volume contains a 750 mg dose?
Concentration = drug ÷ FINAL volume (diluent + powder).
= 3.75 mL
Using the same amoxicillin bottle (100 mL of 250 mg/5 mL with 78 mL water; powder volume 22 mL), how much water would you add to make 200 mg/5 mL instead?
Total drug in the bottle stays the same. New final volume, then subtract the powder volume.
= 103 mL
An azithromycin bottle contains 600 mg; adding 9 mL of water makes 15 mL of 200 mg/5 mL. A 22 lb child needs 10 mg/kg. How many mL? (1 kg = 2.2 lb)
Weight in kg → dose in mg → mL at 40 mg/mL.
= 2.5 mL
A 10,000,000-unit vial has a powder volume of 3.6 mL. How much diluent gives 400,000 units/mL?
Final volume first, then subtract the powder volume.
= 21.4 mL
A 1 g vial gives 250 mg/mL when 3.3 mL of diluent is added. How much diluent is needed for 100 mg/mL?
The powder volume isn't given — work it out from the first set of directions.
= 9.3 mL
⚠️ Tricky
A cefazolin 1 g vial has a powder volume of 0.5 mL. How much diluent must be added to make a concentration of 250 mg/mL?
Final volume from the concentration first, then subtract the space the powder takes.
= 3.5 mL
⚠️ The trap: Adding the full 4 mL (the final volume) gives 4.5 mL and only 222 mg/mL.
⚠️ Tricky
A 35.2 lb child is prescribed cefuroxime 15 mg/kg q12h for 10 days. The suspension is 125 mg/5 mL once reconstituted, and each bottle makes 100 mL. How many bottles must be dispensed? (1 kg = 2.2 lb)
15 mg/kg is PER DOSE, given twice a day. Bottles are whole units.
= 2 bottles
⚠️ The trap: Treating 15 mg/kg as the daily dose gives 96 mL and one bottle — the child runs out halfway through.
⚠️ Tricky
A 1 g vial is reconstituted with 9.2 mL of diluent (powder volume 0.8 mL) to give 100 mg/mL. About 0.2 mL is always left in the vial and needle hub and cannot be withdrawn. How many complete 200 mg doses can be drawn from one vial?
Doses come from the WITHDRAWABLE volume, and only complete doses count.
= 4 doses
⚠️ The trap: Ignoring the dead volume gives 1000 ÷ 200 = 5 doses; the fifth syringe is short.
⚠️ Tricky
A vial contains 1.09 g of a drug's sodium salt, equivalent to 1.0 g of the drug. Its powder volume is 0.6 mL. How much diluent gives a concentration of 200 mg/mL of the drug?
Concentrations are stated as the drug, so use the 'equivalent to' amount.
= 4.4 mL
⚠️ The trap: Using the 1.09 g salt weight gives 5.45 − 0.6 = 4.85 mL.
⚠️ Tricky
A 72.6 lb child is prescribed ceftriaxone 75 mg/kg once daily (maximum 2 g per dose). A 2 g vial (powder volume 1.4 mL) is reconstituted with 18.6 mL of diluent. How many mL should be given? (1 kg = 2.2 lb)
Concentration = drug ÷ FINAL volume. Check the weight-based dose against the maximum.
= 20 mL
⚠️ The trap: Missing the 2 g cap gives 2475 mg = 24.75 mL — more than the vial holds.
🕵️ Spot the mistake
A 2 g vial has a powder volume of 0.8 mL. How much diluent gives 200 mg/mL?
A student wrote
⚠️ Traps examiners set
- Treating diluent volume as final volume (ignoring powder volume).
- Calculating concentration as drug ÷ diluent volume.
- Recalculating the powder volume for a new concentration — it is fixed for that vial.
- Forgetting that 250 mg/5 mL must be turned into mg/mL (50) before dividing.
- Mixing units with mg (or millions of units with units) in the same step.
💬 Explain it in your own words
Why is the powder volume the same no matter how much diluent you add, and how does that let you make concentrations the label never mentions?