PTCB Certified Pharmacy Technician (PTCE)MedicationsHard
A pharmacy technician is preparing a non-sterile compound using a Class A torsion balance. The minimum weighable quantity (MWQ) for this balance is 120 mg. If the prescription calls for 10 mg of an active ingredient, what is the smallest amount of diluent that must be used to ensure an accurate weighing?
- A100 mg
- B110 mg
- C130 mg
- D120 mg
Show answer & explanationAnswer & explanation
Correct answer: B. 110 mg
To ensure an accurate weighing when the desired quantity (10 mg) is below the MWQ (120 mg), the active ingredient must be mixed with enough diluent to create a total quantity that meets or exceeds the MWQ. If the active ingredient is 10 mg, and the total must be at least 120 mg, then 120 mg - 10 mg = 110 mg of diluent is required.
Why the other options are wrong
- A. This would result in a total quantity of 110 mg, which is below the MWQ.
- C. This would mean 130 mg of diluent, resulting in a total of 140 mg, which is more than necessary.
- D. This would mean 120 mg of diluent, resulting in a total of 130 mg, which is more than necessary.
Aliquot Weighing
Aliquot weighing is a technique used when the desired quantity of an ingredient is less than the minimum weighable quantity (MWQ) of the balance. The ingredient is mixed with a diluent to make a larger, weighable quantity, and then a portion (aliquot) of this mixture is used.
- Ensures accuracy when weighing small amounts.
- Total quantity weighed must meet or exceed MWQ.
- Calculation involves ratio and proportion or simply ensuring total weight > MWQ.
Memory trick: Aliquot: 'A little' ingredient needs 'A lot' of diluent to weigh right.