What is the molecular weight of water?

Appeared in: RIMS Nursing Officer - 2022

Explanation

Water's chemical formula is H₂O, meaning it consists of two hydrogen atoms and one oxygen atom.
The atomic weight of hydrogen is approximately 1.008 g/mol, and the atomic weight of oxygen is approximately 15.999 g/mol.
The molecular weight is the sum of the atomic weights of all atoms in the molecule.
Calculation: (2 × 1.008 g/mol for Hydrogen) + (1 × 15.999 g/mol for Oxygen) equals 18.015 g/mol.

Option A: This value (16.015 g/mol) is very close to the atomic weight of a single oxygen atom (15.999 g/mol) and incorrectly omits the mass of the two hydrogen atoms.
Option B: This value (17.015 g/mol) is close to the mass of a hydroxyl radical (OH) or ion (OH⁻), which contains only one hydrogen atom and one oxygen atom (1.008 + 15.999 ≈ 17.007 g/mol).
Option D: This value (20.015 g/mol) is approximately the molecular weight of heavy water (D₂O), where each hydrogen atom is replaced by its heavier isotope, deuterium (D), which has an atomic mass of about 2.014 g/mol. (2 x 2.014) + 15.999 ≈ 20.027 g/mol.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Calculation of the molecular weight of water (H₂O) as background academic context rather than a clinical decision trigger.
The concept of molecular weight is the foundation for calculating molarity (moles/L), a common unit of concentration for IV solutions and medications. Nurses must understand these calculations for safe drug administration.
Knowing that 1 mole of water is about 18 grams (and thus ~18 mL) helps in understanding solution concentrations and osmolarity, which affects fluid shifts between body compartments.
What if? If a nurse were compounding a solution and confused atomic weight with molecular weight (e.g., used 16 g/mol for water instead of 18 g/mol), all subsequent molarity and dosage calculations would be incorrect, potentially leading to a medication error.

How to Approach the Question

First, identify the chemical formula for the substance in question. For water, this is H₂O.
Next, determine the number of atoms for each element in the formula (2 Hydrogen, 1 Oxygen).
Recall or find the standard atomic weight for each element (H ≈ 1.008 g/mol; O ≈ 15.999 g/mol).
Multiply the count of each atom by its atomic weight.
Finally, sum the total weights of all atoms to get the molecular weight.
Compare your calculated result with the given options to select the correct one.

Concept Tested & Keywords

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E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 1 (pp 26-370 of 370) p. 25-27

E6 Physiology Ganong 27e Vol 1 Part 1 p. 13-15

Practise the full RIMS Nursing Officer - 2022

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