If 11 grams of a 10.5% zinc oxide ointment is diluted with 18.5 g of white petrolatum (base), the percentage strength (w/w) of the resulting product is 4.17%.
To calculate this, we first need to find the amount of zinc oxide in the original ointment. This is equal to 11 g * 0.105 = 1.155 g.
We then need to find the total amount of ointment in the new product. This is equal to 11 g + 18.5 g = 29.5 g.
Finally, we divide the amount of zinc oxide by the total amount of ointment to get the percentage strength:
1.155 g / 29.5 g = 0.039 = 4.17%
Therefore, the percentage strength (w/w) of the resulting product is 4.17%.