Question

A pharmacist has a prescription for 30 ml of an eye drop that is to contain benzalkonium chloride 1:750 as a preservative. on the shelf the pharmacist finds 17% benzalkonium chloride. using a dropper that delivers 25 drops/ml, how many drops will be required to prepare this compounded formula? (answer must be numeric; no units or commas; round the final answer to the nearest whole number.)

Answer 1

Let the volume of the 17% solution added be x.
The final volume will be 30 ml with a concentration of 1/750.

The amount of benzalkonium present will remain constant, and is the same in the final solution as in the pharmacists solution. Considering that:
moles = concentration * volume

C₁V₁ = C₂V₂

0.17 * x = 1/750 * 30

x = 0.24 ml

The number of drops that will be added are:
25 * 0.24 = 6

6 drops will be added by the pharmacist.