To determine theweight/weight (w/w) percentage strength, use the formula:

\text{w/w % strength} = \left( \frac{\text{Mass of Active Ingredient (g)}}{\text{Total Mass of Final Product (g)}} \right) \times 100

Given:

Mass of zinc oxide (active ingredient) = 4.5 g

Total mass of final ointment = 30 g

\text{w/w % strength} = \left( \frac{4.5}{30} \right) \times 100 =0.15×100= 0.15 \times 100=0.15×100 =15%= 15\%=15%

(A) 0.25%❌Incorrect – This is too low.

(B) 10%❌Incorrect – This would be true if 3 g of zinc oxide were used in 30 g total.

(C) 15%✅Correct – The calculation confirms this answer.

(D) 17.5%❌Incorrect – This would be true if 5.25 g of zinc oxide were used in 30 g total.

Pharmaceutical Calculations, 15th Edition– Howard C. Ansel

The United States Pharmacopeia (USP) Chapter <795>– Compounding Non-Sterile Preparations

PTCB Exam Content Outline – Pharmaceutical Calculations

Analysis of Answer Choices:References for Pharmacy Technicians: