PEBC Pharmaceutical Calculations 5 — Questions and Answers
Question 1: A medication has a half-life of 6 hours. If a patient receives a 500 mg dose, approximately how much drug remains in the body after 24 hours?
- 31.25 mg (Correct answer)
- 62.5 mg
- 125 mg
- 15.6 mg
Correct answer: 31.25 mg
24 hours = 4 half-lives; 500 × (1/2)^4 = 500 ÷ 16 = 31.25 mg.
Question 2: A total parenteral nutrition (TPN) bag contains 500 mL of 50% dextrose and 500 mL of 10% amino acids. What is the final dextrose concentration in the combined 1000 mL solution?
- 25% (Correct answer)
- 30%
- 20%
- 60%
Correct answer: 25%
Dextrose content = 500 mL × 0.50 = 250 g; final concentration = 250 g ÷ 1000 mL = 0.25 = 25%.
Question 3: A physician orders vancomycin 15 mg/kg IV every 12 hours for a 70 kg patient. The pharmacy has vancomycin 500 mg/10 mL vials. How many mL are needed per dose?
- 21 mL (Correct answer)
- 10.5 mL
- 15 mL
- 30 mL
Correct answer: 21 mL
Dose = 15 mg/kg × 70 kg = 1050 mg; concentration = 500 mg/10 mL = 50 mg/mL; volume = 1050 ÷ 50 = 21 mL.
Question 4: An oral solution contains 125 mg of active ingredient per 5 mL. A patient is prescribed 0.375 g twice daily. How many mL should the patient take per dose?
- 15 mL (Correct answer)
- 7.5 mL
- 30 mL
- 10 mL
Correct answer: 15 mL
0.375 g = 375 mg; volume = (375 mg ÷ 125 mg) × 5 mL = 15 mL per dose.
Question 5: A pharmacy receives an order for potassium chloride 20 mEq IV in 100 mL NS. KCl is available as 2 mEq/mL. How many mL of KCl solution should be added to the bag?
- 10 mL (Correct answer)
- 20 mL
- 5 mL
- 40 mL
Correct answer: 10 mL
Volume = 20 mEq ÷ 2 mEq/mL = 10 mL.
Question 6: A 1:200 w/v solution contains how many mg of drug per mL?
- 5 mg/mL (Correct answer)
- 0.5 mg/mL
- 50 mg/mL
- 2 mg/mL
Correct answer: 5 mg/mL
1:200 w/v = 1 g per 200 mL = 1000 mg ÷ 200 mL = 5 mg/mL.
Question 7: A patient needs phenytoin 5 mg/kg/day in 2 divided doses. The patient weighs 154 lbs. Phenytoin suspension is 125 mg/5 mL. How many mL per dose?
- 7 mL (Correct answer)
- 14 mL
- 3.5 mL
- 5 mL
Correct answer: 7 mL
Weight = 154 lbs ÷ 2.2 = 70 kg; daily dose = 5 × 70 = 350 mg; per dose = 175 mg; volume = (175/125) × 5 = 7 mL.
PEBC Pharmaceutical Calculations 5
A medication has a half-life of 6 hours.
If a patient receives a 500 mg dose, approximately how much drug remains in the body after 24 hours?