PTCB Certified Pharmacy Technician (PTCE)Order Entry and ProcessingMedium
A pharmacy technician is calculating the amount of active ingredient needed to compound 120 mL of a 2.5% (w/v) solution. How many grams of active ingredient are required?
- A0.3 g
- B30 g
- C3 g
- D2.5 g
Show answer & explanationAnswer & explanation
Correct answer: C. 3 g
A 2.5% (w/v) solution means 2.5 grams of active ingredient per 100 mL of solution. To find the amount for 120 mL: (2.5 g / 100 mL) = (x g / 120 mL). Solving for x: x = (2.5 * 120) / 100 = 300 / 100 = 3 grams.
Why the other options are wrong
- A. Incorrect; this would be for a 0.25% solution or 12 mL of a 2.5% solution.
- B. Incorrect; this would be for a 25% solution or 1200 mL of a 2.5% solution.
- D. Incorrect; this is the percentage value, not the amount for 120 mL.
Percent Weight-in-Volume (w/v) Calculation
A calculation to determine the amount (in grams) of a solid active ingredient needed to prepare a specified volume of solution at a given percent weight-in-volume concentration.
- % w/v means grams per 100 mL.
- Use ratio-proportion or dimensional analysis.
- Units must be consistent.
Memory trick: Grams per Hundred, Then Adjust for Volume Wonder!