Question

The pharmacy receives a prescription for 50 grams of 5% salicylic acid ointment. the pharmacy has a 30% salicylic acid ointment and petrolatum available. using only these products, how much petrolatum should be used to compound the prescription? express the answer in grams. (answer must be numeric; no units or commas; include leading zero when answer is less than 1; round the final answer to the nearest one decimal place.)

Answer 1

In this problem, we can formulate two equations.

First, the overall mass balance of the ointment. The mass of 30% salicylic acid ointment plus the mass of petrolatum must be 50 grams.

x + p = 50

Where x is the mass of 30% salicylic acid ointment and p is the mass of petrolatum

Second, the component mass balance of salicylic acid:

0.3 * x = 50 * 0.05

x = 8.33 g

Therefore the mass of petrolatum to be used is:

x + p = 50

p = 50 – x

p = 50 – 8.33

p = 41.67 g

 

Answer:

41.7