Question

You have been asked to mix 10 mL of fortified tobramycin eye drops (1%) for a patient. You have the commercially prepared Tobrex 0.3% 10 mL on your shelf. You also have some preservative-free tobramycin 40 mg/mL solution. Explain how you will mix this prescription. Show all steps to arrive at the completed summary statement below. 
I will withdraw____________mL of Tobrex solution from the Tobrex container and replace it with____________mL of tobramycin 40 mg/mL solution. 


Please show all work clearly

Answer 1

You want to make 10 mL 1% solution. You have 10 mL 0.3% solution and 40 mg/mL solution at your disposal.

10 mL 1% solution will contain (1 g)*(10 mL/100 mL) = 0.1 g Tobramycin (remember that 1% solution means 1 g solute in 100 g solvent).

10 mL 0.3% solution contains (0.3 g)*(10 mL/100 mL) = 0.03 g Tobrex.

40 mg/mL solution means 1 mL solution contains 40 mg = (40 mg)*(1 g/1000 mg) = 0.04 g Tobramycin.

Let us take 10 mL 0.3% Tobrex solution from the shelf and add x mL of 40 mg/mL preservative-free Tobramycin solution.

Therefore, weigh of Tobramycin taken = (x mL)*(0.04 g/1 mL) = 0.04x Tobramycin.

Set up the equation as

0.03 g + 0.04x = 0.1

===> 0.04x = 0.07

===> x = 0.07/0.04 = 1.75

Therefore, you will need to withdraw 10 mL 0.3% Tobarex solution from the shelf and add 1.75 mL of 40 mg/mL Tobramycin solution to prepare 10 mL of 1% Tobramycin eye-drop (ans).