A syringe with a concentration of 3 mg/mL is used to deliver 9 mg. How many milliliters must be drawn?

A syringe with a concentration of 3 mg/mL is used to deliver 9 mg. How many milliliters must be drawn?

• A. 2 mL
• B. 3 mL
• C. 4 mL
• D. 1.5 mL

Answer: (B)

Key idea: use the relationship dose = concentration × volume to solve for volume. If you need 9 mg and the syringe delivers 3 mg per milliliter, the volume required is 9 mg ÷ (3 mg/mL) = 3 mL. This checks out because 3 mg/mL times 3 mL equals 9 mg. To see why other amounts don’t fit, note that 2 mL would deliver 6 mg and 4 mL would deliver 12 mg, not the target. The key is canceling units so mg cancels and you’re left with milliliters.

Key idea: use the relationship dose = concentration × volume to solve for volume. If you need 9 mg and the syringe delivers 3 mg per milliliter, the volume required is 9 mg ÷ (3 mg/mL) = 3 mL. This checks out because 3 mg/mL times 3 mL equals 9 mg. To see why other amounts don’t fit, note that 2 mL would deliver 6 mg and 4 mL would deliver 12 mg, not the target. The key is canceling units so mg cancels and you’re left with milliliters.