Isotonicity Naplex Practice Problema

Caroline Mae

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01-01-2025

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The NAPLEX (North American Pharmacist Licensure Examination) demands a thorough understanding of pharmaceutical calculations, including isotonicity. Failing to grasp this concept can significantly impact your score. Let's delve into some practice problems to solidify your understanding.

What is Isotonicity?

Isotonicity refers to the osmotic pressure exerted by a solution compared to that of body fluids (typically blood plasma or tears). Solutions that have the same osmotic pressure as body fluids are considered isotonic. Administering a hypotonic solution (lower osmotic pressure) can cause cells to swell and potentially lyse (burst), while a hypertonic solution (higher osmotic pressure) can cause cells to shrink. Maintaining isotonicity is crucial for patient safety and comfort, especially with intravenous or ophthalmic solutions.

Practice Problems

Let's tackle a few NAPLEX-style problems focusing on isotonicity calculations:

Problem 1:

A pharmacist needs to prepare 100 mL of an isotonic solution of drug X. The drug X is known to have a freezing point depression of -0.52°C. The freezing point depression of a 0.9% w/v sodium chloride solution (isotonic) is -0.52°C. How much drug X should be added to achieve isotonicity in 100 mL of water?

Solution:

Since the freezing point depression of drug X is equivalent to that of an isotonic sodium chloride solution, the same amount of drug X will result in an isotonic solution. The problem provides no information regarding the concentration of the drug, making it impossible to calculate precisely how much drug X should be used. More information is needed to solve this problem.

Problem 2:

You need to prepare 50 mL of an isotonic solution of boric acid. The freezing point depression of a 1% w/v boric acid solution is -0.29°C. The freezing point depression of an isotonic NaCl solution (0.9% w/v) is -0.52°C. Using the freezing point depression method, calculate the amount of boric acid required.

Solution:

This problem requires using the following formula:

• Weight of solute (boric acid) = (Freezing point depression of boric acid solution/Freezing point depression of isotonic NaCl solution) * Weight of isotonic NaCl solution needed

First, we determine the weight of isotonic NaCl needed for 50 mL: (0.9g/100mL) * 50mL = 0.45g

Then, we apply the formula:

Weight of boric acid = (-0.29°C / -0.52°C) * 0.45g ≈ 0.25g

Therefore, approximately 0.25g of boric acid is needed to prepare 50 mL of an isotonic solution.

Problem 3:

A physician orders 100 mL of a 2% solution of dextrose. Dextrose has an E value of 0.16. Calculate the amount of dextrose needed, and discuss the isotonicity of this solution relative to physiological saline.

Solution:

This problem utilises the E value method. The E value is a factor that takes into account the osmotic pressure contributions of different molecules. We can use this formula:

• Weight of Solute = desired volume (mL) x desired concentration (%) x E value

Weight of Dextrose = 100 mL * 2% * 0.16 = 3.2g

Therefore, 3.2g of dextrose is required. A 2% dextrose solution is not isotonic with physiological saline. It will be hypotonic compared to physiological saline and thus would have different osmotic effects on cells.

Key Takeaways

Mastering isotonicity calculations is vital for the NAPLEX. Understanding the principles behind freezing point depression and the E value method is crucial for accurate preparation of solutions for patient administration. Practice consistently using various problems, and don't hesitate to seek additional resources if needed. Remember to always double-check your calculations!