Final answer:
24 grams of zinc oxide powder is required to prepare 120 g of a 20% w/w ointment based on a simple proportionate calculation.
Step-by-step explanation:
To prepare a 120 g of 20% w/w ointment of zinc oxide, you need to calculate the amount of zinc oxide based on the total weight of the ointment. A 20% w/w ointment means that 20 g of zinc oxide are present in every 100 g of the ointment. To find the amount needed for 120 g of ointment, you can set up a simple proportion:
(20 g zinc oxide / 100 g ointment) = (x g zinc oxide / 120 g ointment)
Cross-multiplying to solve for x, you'll get:
20 g * 120 g = 100 g * x
2400 = 100x
x = 24 g
Therefore, 24 grams of zinc oxide powder is required to prepare 120 g of a 20% w/w ointment.