Using a bottle labeled 250 mg/5 mL, how many milliliters are needed for a 500 mg dose?

• A. 5 mL
• B. 7.5 mL
• C. 10 mL
• D. 15 mL

Answer: (C)

The key idea is converting a dose to a volume using the concentration. If a bottle has 250 mg in 5 mL, that’s 250 ÷ 5 = 50 mg per mL. To deliver 500 mg, divide 500 mg by 50 mg/mL, which equals 10 mL. You can also see this by proportion: doubling the dose from 250 mg to 500 mg doubles the volume from 5 mL to 10 mL. Check: 10 mL × 50 mg/mL = 500 mg. So the required volume is 10 mL.