Medium NAPLEX Compounding Practice Questions

Concept Explanation

NAPLEX compounding refers to the professional practice of preparing, mixing, assembling, packaging, or labeling a drug or device as a result of a practitioner's prescription drug order based on the practitioner/patient/pharmacist relationship. To succeed, candidates must master pharmaceutical calculations, including percentage strength, ratio strength, and alligation. Compounding requires strict adherence to United States Pharmacopeia (USP) standards, specifically USP Chapters <795> for non-sterile and <797> for sterile preparations.

Solved Examples

1. Calculate the amount of active ingredient needed to prepare 500 mL of a 2% (w/v) solution.
   A 2% (w/v) solution contains 2 grams per 100 mL. Using the ratio method: $$\ \frac{2 \ \text{ g}}{100 \ \text{ mL}} = \ \frac{x \ \text{ g}}{500 \ \text{ mL}}$$ $$x = \ \frac{2 \ \times 500}{100} = 10 \ \text{ grams}$$
2. How many grams of a 10% ointment and a 2% ointment are required to prepare 100 g of a 5% ointment?
   Using alligation:
   10% (High) | 3 parts (5-2)
   5% (Target)
   2% (Low) | 5 parts (10-5)
   Total parts = 8.
   For 10% ointment: $\ \frac{3}{8} \ \times 100 \ \text{ g} = 37.5 \ \text{ g}$.
   For 2% ointment: $\ \frac{5}{8} \ \times 100 \ \text{ g} = 62.5 \ \text{ g}$.
3. A pharmacist needs to prepare 60 mL of a 1:500 solution from a 1:100 stock solution. How many mL of stock solution are needed?
   Use the formula $C_1V_1 = C_2V_2$. Convert ratios to decimals: 1:500 = 0.002, 1:100 = 0.01. $$0.01 \ \times V_1 = 0.002 \ \times 60$$ $$V_1 = \ \frac{0.12}{0.01} = 12 \ \text{ mL}$$

Practice Questions

1. How many milligrams of hydrocortisone are needed to prepare 30 g of a 0.5% (w/w) ointment?
2. A pharmacist is asked to prepare 240 mL of a solution containing 0.25% of drug X. How many mg of drug X are required?
3. You have a 50% dextrose solution and sterile water. How much of each is needed to make 500 mL of a 10% dextrose solution?

4. Calculate the percentage strength of a solution prepared by mixing 200 mL of a 10% solution with 300 mL of a 5% solution.
5. How many mL of a 1:200 w/v solution can be prepared from 5 g of drug?
6. If a compounded cream contains 15 g of active ingredient in 120 g of total product, what is the percentage strength (w/w)?
7. A pharmacist adds 50 mL of water to 150 mL of a 20% solution. What is the new concentration?
8. How many grams of zinc oxide are required to prepare 1 lb (454 g) of a 5% ointment?
9. A formula requires 0.5 g of a potent drug. You have a 1:100 trituration. How many grams of the trituration should be used?
10. What is the ratio strength of a 0.05% solution?

Answers & Explanations

1. 150 mg. (0.5% = 0.5g/100g. 0.005 * 30g = 0.15g = 150mg).
2. 600 mg. (0.25% = 0.25g/100mL. 0.0025 * 240mL = 0.6g = 600mg).
3. 100 mL of 50% dextrose and 400 mL of sterile water. (Using $C_1V_1 = C_2V_2$: $50 \ \times V_1 = 10 \ \times 500 \ \rightarrow V_1 = 100 \ \text{ mL}$).
4. 7%. ((200 * 0.10) + (300 * 0.05) = 20 + 15 = 35g total. 35g / 500mL = 0.07 = 7%).
5. 1,000 mL. (1:200 = 0.5g/100mL. 5g / 0.005 = 1,000 mL).
6. 12.5%. (15g / 120g = 0.125 = 12.5%).
7. 15%. (150 * 0.20 = 30g drug. 30g / 200mL total = 0.15 = 15%).
8. 22.7 g. (454 * 0.05 = 22.7g).
9. 50 g. (1:100 means 1g drug per 100g trituration. 0.5g drug * 100 = 50g trituration).
10. 1:2,000. (0.05% = 0.05/100 = 1/2000).

Quick Quiz

Frequently Asked Questions

What is the difference between w/v and w/w?

Weight-in-volume (w/v) expresses grams of solute per 100 mL of solution, which is common for liquids. Weight-in-weight (w/w) expresses grams of solute per 100 grams of the final product, typically used for creams and ointments.

How do I determine the amount of drug needed for a trituration?

A trituration is a dilution of a potent drug with an inert substance. Multiply the desired amount of the active drug by the inverse of the trituration ratio to find the required weight of the trituration mixture.

Why is alligation used in compounding?

Alligation is a shortcut method for calculating the proportions of two substances of different concentrations required to create a specific final concentration. It simplifies algebraic systems of equations into a visual grid format.

What is the standard weight/volume relationship for percentage strength?

A percentage strength of 1% is defined as 1 gram of solute per 100 units of the total preparation. This applies to both solids (g/100g) and liquids (g/100mL).

Are NAPLEX compounding questions mostly calculation-based?

While some questions test regulatory knowledge of USP chapters, a significant portion of compounding questions focus on accurate pharmaceutical calculations. Mastery of ratios, proportions, and unit conversions is essential for these tasks.

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