At 25 °C, a solution has equal concentrations of hydrogen ions [H+] and hydroxide ions [OH−]. What will be the pH of this solution?

Introduction / Context:
The pH scale quantifies acidity via hydrogen ion concentration, while pOH relates to hydroxide ions. At 25 °C, the ion product of water Kw equals 1.0 × 10^-14, leading to a convenient relationship between pH and pOH. Understanding neutrality and the meaning of equal [H+] and [OH−] is foundational in aqueous chemistry, biochemistry, and environmental engineering.

Given Data / Assumptions:

• Temperature is 25 °C (298 K).
• Neutral water satisfies [H+][OH−] = Kw = 1.0 × 10^-14.
• The solution has [H+] = [OH−].

Concept / Approach:
If [H+] = [OH−] and their product equals Kw, then each must be 1.0 × 10^-7 M. pH is defined as −log10[H+]. Therefore, pH = 7. This is the conventional neutrality point at 25 °C. Note that neutrality shifts slightly with temperature since Kw depends on T; however, for the stated 25 °C, pH 7 is exactly neutral.

Step-by-Step Solution:

Verification / Alternative check:
pOH = −log10[OH−] = 7; pH + pOH = 14 at 25 °C, matching the standard relation derived from Kw.

Why Other Options Are Wrong:

• 0 or 14 are extreme acid/base values, not neutral.
• 10 or 5 indicate basic or acidic conditions, respectively, which contradict equal [H+] and [OH−].

Common Pitfalls:
Forgetting that neutrality is temperature-dependent; at temperatures other than 25 °C, neutral pH is not exactly 7, but here 7 is correct by definition.

Final Answer:
7