A dose of 75 mg is needed. The stock solution is 100 mg/5 mL. How many mL should be administered?

A dose of 75 mg is needed. The stock solution is 100 mg/5 mL. How many mL should be administered?

• A. 1.5 mL
• B. 7.5 mL
• C. 15 mL
• D. 3.75 mL

Answer: (D)

The amount to administer depends on the concentration. The stock solution has 100 mg in 5 mL, which is 100 mg ÷ 5 mL = 20 mg per mL. To give 75 mg, use volume V where 20 mg/mL × V = 75 mg. Solve for V: V = 75 ÷ 20 = 3.75 mL. A quick check: at 20 mg/mL, 5 mL would be 100 mg, so 75 mg is three-quarters of 5 mL, which is 0.75 × 5 = 3.75 mL. Therefore, administer 3.75 mL.

The amount to administer depends on the concentration. The stock solution has 100 mg in 5 mL, which is 100 mg ÷ 5 mL = 20 mg per mL. To give 75 mg, use volume V where 20 mg/mL × V = 75 mg. Solve for V: V = 75 ÷ 20 = 3.75 mL. A quick check: at 20 mg/mL, 5 mL would be 100 mg, so 75 mg is three-quarters of 5 mL, which is 0.75 × 5 = 3.75 mL. Therefore, administer 3.75 mL.