PTCB Certified Pharmacy Technician (PTCE)MedicationsHard
A pharmacy technician is preparing a non-sterile compound that requires the use of a Class III Torsion balance to weigh an ingredient. The minimum weighable quantity (MWQ) for this balance is 120 mg. If the prescription requires 10 mg of the ingredient, what is the SMALLEST amount of diluent powder (e.g., lactose) that should be added to meet the MWQ for accurate weighing?
- A110 mg
- B10 mg
- C240 mg
- D120 mg
Show answer & explanationAnswer & explanation
Correct answer: A. 110 mg
The minimum weighable quantity (MWQ) for a Class III Torsion balance is 120 mg. To accurately weigh 10 mg of the ingredient, the total weight on the balance must be at least 120 mg. Therefore, 120 mg (MWQ) - 10 mg (ingredient) = 110 mg of diluent powder should be added to reach the MWQ for accurate weighing. This is known as aliquot weighing.
Why the other options are wrong
- B. This is the amount of active ingredient, not the diluent needed to meet MWQ.
- C. This amount would exceed the MWQ unnecessarily if only 10 mg of active ingredient is needed for the aliquot.
- D. This is the MWQ, but the question asks for the amount of diluent to add to the 10 mg ingredient.
Aliquot Weighing
A technique used to accurately weigh quantities below the minimum weighable quantity (MWQ) of a balance by adding a calculable amount of diluent to reach the MWQ, then using a portion of this mixture.
- Ensures accuracy for small quantities.
- Requires diluent (e.g., lactose) to reach MWQ.
- Involves calculating the proportion of active ingredient in the mixture.
Memory trick: MWQ Minus My Tiny Quantity Equals My Diluent.