According to Brønsted–Lowry acid–base theory, what is the conjugate base formed when H2O donates a proton?

Introduction / Context:
This question applies Brønsted Lowry acid base theory, which defines acids as proton donors and bases as proton acceptors. When an acid loses a proton, the species that remains is called its conjugate base. Water is amphoteric, meaning it can act as either an acid or a base. The question asks specifically for the conjugate base of H2O when it donates a proton, a common concept in acid base equilibrium problems.

Given Data / Assumptions:

• H2O is neutral water.
• When H2O donates a proton (H+), it loses one hydrogen ion.
• The remaining species must have one fewer hydrogen and carry a negative charge.
• The options include NH3+, OH-, H2O, and a statement suggesting all of them.

Concept / Approach:
According to Brønsted Lowry theory, an acid base conjugate pair differs by exactly one proton. If H2O acts as an acid and donates one proton, the equation can be written as H2O → H+ + OH-. The species formed after loss of H+ is OH-, the hydroxide ion. Therefore, OH- is the conjugate base of H2O in this context. Ammonium ion NH4+ and ammonia NH3 form another conjugate acid base pair, but NH3+ is not the species formed from water. H2O itself is the original acid in this situation, not its own conjugate base.

Step-by-Step Solution:
Step 1: Write the acid dissociation of water. H2O can donate a proton: H2O → H+ + OH-. Step 2: Identify the species before and after proton donation. Before donation: H2O as the acid. After donation: OH- is left behind. Step 3: Apply the conjugate base definition. Conjugate base = acid minus one proton, so the conjugate base of H2O is OH-.

Verification / Alternative check:
Consider the autoionisation of water: 2H2O ⇌ H3O+ + OH-. In this equilibrium, one water molecule acts as an acid and donates a proton to another water molecule, which acts as a base. The donor becomes OH-, and the acceptor becomes H3O+. Thus, OH- is always observed as the result of water losing a proton. Many acid base tables list conjugate pairs such as H2O and OH-, further confirming that hydroxide ion is the conjugate base of water.

Why Other Options Are Wrong:
Option A: NH3+ is not formed from H2O and is not the conjugate base of water; it would be associated with a different acid base system. Option C: H2O is the original acid in this case, not the species formed after losing a proton, so it cannot be its own conjugate base. Option D: All the above is impossible because only OH- matches the definition of the conjugate base of H2O.

Common Pitfalls:
Learners sometimes confuse conjugate acid and conjugate base pairs or forget that they differ by exactly one proton. Another mistake is to think that any negatively charged species can be a conjugate base of water, which is not correct. Always write the simple equation showing the acid donating a proton and then identify what remains. For water, this process clearly leads to the hydroxide ion as the conjugate base.

Final Answer:
When H2O donates a proton, its conjugate base is OH-.