How much salt must be added to one litre of water to make a solution of normal saline?

Appeared in: VSSC Nursing Officer - 2021

Explanation

Normal saline is defined as a 0.9% weight/volume (w/v) solution of sodium chloride (NaCl).
This concentration means there are 0.9 grams of NaCl for every 100 mL of water.
To prepare 1 litre (1000 mL), the calculation is (0.9 g / 100 mL) * 1000 mL, which equals 9 grams.

Option A: This value is incorrect and does not correspond to any standard intravenous fluid concentration. It is a distractor without a direct calculative basis.
Option B: This is the correct amount of salt for 100 mL of normal saline, not for 1 litre (1000 mL). This is a common error if the volume conversion from mL to L is missed.
Option D: This amount would create a 9% NaCl solution, which is a highly concentrated hypertonic solution. Administering this intravenously would be extremely dangerous and potentially fatal.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Calculation of solute for normal saline solution as background academic context rather than a clinical decision trigger.
Accurate medication and fluid calculation is a fundamental nursing responsibility to ensure patient safety and prevent medication errors.
Administering an incorrect concentration, such as a 9% solution (90g/L), would be severely hypertonic, causing massive fluid shifts, cellular dehydration, and potentially fatal complications like osmotic demyelination syndrome.
What if? If a nurse needs to prepare 250 mL of normal saline to dilute a medication, they would need to add (0.9g / 100mL) * 250mL = 2.25 grams of salt.

How to Approach the Question

Identify the core question: calculating the amount of solute (salt) for a specific volume (1 litre) of a known solution (normal saline).
Recall the standard definition of normal saline: 0.9% NaCl.
Translate the percentage concentration into a measurable unit: 0.9% w/v means 0.9 grams per 100 mL.
Convert the required volume to the same unit as the definition: 1 litre = 1000 mL.
Perform the calculation: If 100 mL needs 0.9 g, then 1000 mL needs 10 times that amount (0.9 g x 10).
Select the option that matches the calculated result (9 g).

Concept Tested & Keywords

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E6 Pharmacology Nursing Lilley 11e Part 2 p. 161-163

E6 Medicine Davidson Principles Practice 24e p. 373-375

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