The osmolality of 0.45% Sodium Chloride (NaCl) is:

Appeared in: GMCH - 2016

Explanation

The osmolality of an IV solution refers to the concentration of solute particles within it.
9% NaCl (Normal Saline) is the standard isotonic solution, with an osmolality of approximately 308 mOsm/L, which is similar to human plasma.
45% NaCl is known as half-normal saline because it has half the concentration of solutes as normal saline.
Therefore, its osmolality is half that of 0.9% NaCl: 308 mOsm/L / 2 = 154 mOsm/L.
This lower concentration makes it a hypotonic solution, meaning it will cause water to shift into cells.

Option B: This value is incorrect for 0.45% NaCl. An osmolality of 278 mOsm/L is characteristic of Lactated Ringer's (LR) solution.
Option C: This is the osmolality of 0.9% NaCl (Normal Saline), not 0.45% NaCl. It represents an isotonic solution.
Option D: This value represents a hypertonic solution. An osmolality of 512 mOsm/L is much higher than that of 0.45% NaCl.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Osmolality and Tonicity of Intravenous (IV) Solutions as background academic context rather than a clinical decision trigger.
Understanding the osmolality and tonicity of IV fluids is critical for safe nursing practice to prevent fluid shifts, electrolyte imbalances, and complications like cerebral edema or cellular dehydration.
Nurses must select the appropriate IV fluid based on the patient's clinical condition, such as using hypotonic fluids for cellular dehydration (e.g., diabetic ketoacidosis after initial stabilization) and isotonic fluids for volume expansion.
What if? If a patient with a head injury and suspected increased intracranial pressure (ICP) were mistakenly given 0.45% NaCl, the hypotonic nature of the fluid would cause water to shift into the brain cells, worsening cerebral edema and potentially leading to brain herniation. In this case, an isotonic or even hypertonic solution would be indicated.

How to Approach the Question

This is a factual recall question that tests your knowledge of common intravenous solutions.
First, identify the substance in question: 0.45% Sodium Chloride (NaCl).
Recall the common name for this solution: 'half-normal saline'.
Use 'normal saline' (0.9% NaCl) as a reference point. Remember that 0.9% NaCl is isotonic and has an osmolality of about 308 mOsm/L.
Since 0.45% is half of 0.9%, calculate half the osmolality of normal saline: 308 / 2 = 154 mOsm/L.
Compare your calculated value with the given options to find the correct answer.

Concept Tested & Keywords

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E6 Physiology Ganong 27e Vol 1 Part 1 p. 16-18

E6 Physiology Guyton 4SAE Part 1 p. 78-80

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Attempt every question from this paper in a timed mock, then review the full solution for each one.