A vial contains 2 mg/mL. If the order is 8 mg, how many milliliters are required?

A vial contains 2 mg/mL. If the order is 8 mg, how many milliliters are required?

• A. 2 mL
• B. 8 mL
• C. 4 mL
• D. 0.5 mL

Answer: (C)

When you have a concentration, volume and dose relate by Volume = Dose / (Concentration). The vial has 2 mg per 1 mL, so to get 8 mg you divide 8 mg by 2 mg/mL: 8 ÷ 2 = 4 mL. Check: 4 mL × 2 mg/mL = 8 mg, which matches the order. The other amounts would yield different doses (2 mL would be 4 mg, 8 mL would be 16 mg, 0.5 mL would be 1 mg), so 4 mL is the correct volume.

When you have a concentration, volume and dose relate by Volume = Dose / (Concentration). The vial has 2 mg per 1 mL, so to get 8 mg you divide 8 mg by 2 mg/mL: 8 ÷ 2 = 4 mL. Check: 4 mL × 2 mg/mL = 8 mg, which matches the order. The other amounts would yield different doses (2 mL would be 4 mg, 8 mL would be 16 mg, 0.5 mL would be 1 mg), so 4 mL is the correct volume.