You have been asked to compound 30 g of a 10% w/w zinc oxide cream. You only hav

Question

You have been asked to compound 30 g of a 10% w/w zinc oxide cream. You only have 40% w/w zinc oxide cream in stock. How much grams of this 40% stock cream will you mix with pure cream base to make the compound?

A 5-million-unit vial of penicillin G potassium is to be reconstituted with 8.2 mL of diluent to achieve a concentration of 500,000 units/mL. How many mL of diluent should be used to instead reconstitute the same vial to a concentration of 750,000 units/mL? Round your answer to the nearest mL.

Explanation / Answer

1. 7.5 g of 40% zinc oxide

Requirement =

100 g of 10% w/w zinc oxide contains, 10 g of zinc oxide

So 30 g of a 10% w/w zinc oxide contains, 30 x 10 = 300 g of zinc oxide

Stock =

100 g of 40%   w/w zinc oxide contains, 40 g of zinc oxide.

The amount of 40%   w/w zinc oxide required to make 300 g of zinc oxide = 300 / 40 = 7.5 g

2. Amount of diluent = 4.87 mL

5-million-unit vial of penicillin G potassium + 8.2 mL of diluent = 500,000 units/mL.

5,000,000 units is being diluted to 500,000/mL, indicating a 10 times dilution, so the actual volume of penicillin G potassium in the vial = 10 - 8.2 = 1.8 mL

i.e. initially there was 1.8 mL of 5-million-unit penicillin, to make 750,000 units/mL, = 5000000/750000 = 6.6667 times dilution needed to be made, i.e. you have to add 6.6667 - 1.8 = 4.8667 mL of diluent.

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