Ampicillin 250 mg IV every 6 hours; stock is 1000 mg/50 mL. How many mL are needed for a 125 mg dose?

Ampicillin 250 mg IV every 6 hours; stock is 1000 mg/50 mL. How many mL are needed for a 125 mg dose?

• A. 6 mL
• B. 6.25 mL
• C. 7.5 mL
• D. 8 mL

Answer: (B)

Think in terms of concentration and volume. The stock solution has 1000 mg in 50 mL, which is 1000 ÷ 50 = 20 mg per mL. To deliver 125 mg, use the volume that provides that amount: 125 mg ÷ 20 mg/mL = 6.25 mL. You can also see this with a cross‑multiply setup: 125 mg corresponds to x mL, and 1000 mg corresponds to 50 mL, so x = (125 × 50) / 1000 = 6.25 mL. So you would administer 6.25 mL of the stock solution to give a 125 mg dose.

Think in terms of concentration and volume. The stock solution has 1000 mg in 50 mL, which is 1000 ÷ 50 = 20 mg per mL. To deliver 125 mg, use the volume that provides that amount: 125 mg ÷ 20 mg/mL = 6.25 mL. You can also see this with a cross‑multiply setup: 125 mg corresponds to x mL, and 1000 mg corresponds to 50 mL, so x = (125 × 50) / 1000 = 6.25 mL. So you would administer 6.25 mL of the stock solution to give a 125 mg dose.