Which of the following is the conjugate base of water?

Appeared in: NHM UP Staff Nurse-2023 Shift-2nd

Explanation

According to the Brønsted-Lowry theory, an acid is a substance that donates a proton (H⁺).
Water (H₂O) can act as an acid by donating one of its protons.
When a water molecule (H₂O) donates a proton (H⁺), the remaining ion is the hydroxide ion (OH⁻).
Therefore, the hydroxide ion (OH⁻) is the conjugate base of water.

Option A: Oxygen is a chemical element, not an ion formed by the loss of a proton from water.
Option B: Oxyheme is a component of hemoglobin involved in oxygen transport and is not related to the acid-base chemistry of water.
Option D: Hydride (H⁻) is an anion formed when a hydrogen atom gains an electron. The conjugate base of water is formed when it loses a proton (H⁺).

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Brønsted-Lowry Acid-Base Theory as background academic context rather than a clinical decision trigger.
Understanding conjugate acid-base pairs is fundamental to grasping how the body's buffer systems, like the bicarbonate buffer system (H₂CO₃/HCO₃⁻), maintain blood pH.
Nurses must understand acid-base balance to interpret arterial blood gas (ABG) results and recognize conditions like metabolic or respiratory acidosis and alkalosis.
What if the question asked for the conjugate acid of water? The answer would be the hydronium ion (H₃O⁺), which is formed when water acts as a base and accepts a proton (H₂O + H⁺ → H₃O⁺).

How to Approach the Question

First, recall the Brønsted-Lowry definition of an acid (proton donor) and a base (proton acceptor).
Understand that a 'conjugate base' is what remains after an acid has donated its proton.
Apply this concept to a water molecule (H₂O) acting as an acid.
Visualize the water molecule losing a proton (H⁺). The remaining part is OH⁻.
Identify OH⁻ as the hydroxide ion and select the corresponding option.
Eliminate the other options by identifying them as an element (Oxygen), a complex biological molecule (Oxyheme), or an incorrect ion (Hydride).

Concept Tested & Keywords

QKp8DU7gYnNgVBxoSrbOKl

E6 Physiology Guyton 4SAE Part 2B p. 65-67

Practise the full NHM UP Staff Nurse-2023 Shift-2nd

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