What is the total amount of drug present in the final 50 mL solution?

• A. 25 mg
• B. 50 mg
• C. 12.5 mg
• D. 100 mg

Answer: (A)

The amount of drug in the final solution is found by multiplying the final concentration by the final volume, because when you dilute a solution the same total amount of drug is spread out over a larger volume. If the final 50 mL solution has a concentration of 0.5 mg/mL, the total drug present is 0.5 mg/mL × 50 mL = 25 mg. So 25 mg is the total amount in that 50 mL solution. If the concentration were different, the total would change accordingly (for example, a concentration of 1 mg/mL would give 50 mg, etc.), but given the final concentration in this scenario, 25 mg is the correct total.