How many grams of zinc oxide should be added to 3400 g of a 10% zinc oxide ointment to prepare a product containing 15% of zinc oxide?

10% w/w = 10g/ 100g = x /3400g

x = amount of drug in 3400 g

or i would just say 10% of 3400 = 340 g of zinc oxide is needed to be 10% strength

now how much do you have to add of zinc oxide to get 15% strength??

so you will end up with 340g of original PLUS x amount of new zinc oxide = the total drug you will use.

so (340 + x)

and to find percentage of drug over entire medication ... you would say (340 + x) over (3400 + x)

and (3400 + x) = total medication with the additional zinc oxide you will want to add.

finally: total drug/ total medication(Rx) = the new percentage strength

sooo --> you wanted 15% to be the zinc oxide (drug)

(340 + x) / (3400 + x) = 15% drug

or better yet: (340 + x) / (3400 + x) = 15g/ 100g

since 15% is 15 out of 100

so CROSS MULTIPLY

and solve for x:

(340 + x) * 100 = (3400 + x) * 15

34000 + 100 x = 51000 + 15 x--> 85 x = 17000

x = 200 so 200 grams of zinc oxide must be added to the original to get your 15% of drug strength.

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