How much oral phenobarbital solution should a nurse administer if the prescribed dose is 30 mg and the available concentration is 20 mg/4 mL?

To calculate the required volume of the phenobarbital solution, you first need to determine how many milliliters contain the prescribed dose of 30 mg based on the available concentration of 20 mg in 4 mL.

First, find out how many milligrams are present in 1 mL of the solution. Since the concentration is 20 mg per 4 mL, you can divide the amount of medication by the volume to find the concentration per mL:

20 mg / 4 mL = 5 mg/mL.

Next, to find out how many milliliters are needed to achieve a dose of 30 mg, divide the prescribed dose by the concentration per mL:

30 mg ÷ 5 mg/mL = 6 mL.

This calculation shows that to obtain a dose of 30 mg, the nurse should administer 6 mL of the phenobarbital solution. Thus, the correct answer is based on the calculation aligning the prescribed dose with the concentration available, confirming that 6 mL will provide the necessary 30 mg of medication.