Pure distilled water at 25°C: What is the hydrogen ion concentration [H+] in mol/L under neutral conditions?

Introduction:
Neutral water has equal hydrogen and hydroxide ion concentrations. At standard temperature, this defines the pH scale midpoint.

Given Data / Assumptions:

• Distilled water, 25°C assumed.
• Neutral condition (pH 7).
• Units in mol/L.

Concept / Approach:
At 25°C, water autoionization constant Kw = [H+] * [OH-] = 1e-14. Neutrality implies [H+] = [OH-] = sqrt(1e-14) = 1e-7 mol/L.

Step-by-Step Solution:
Step 1: Use Kw = 1e-14 at 25°C.Step 2: For neutral water, [H+] = [OH-].Step 3: [H+] = sqrt(1e-14) = 1e-7 mol/L.

Verification / Alternative check:
pH = -log10([H+]) = 7 confirms neutrality.

Why Other Options Are Wrong:
7e-7 overstates [H+]; 1e7 and 7e7 are impossible; 1e-5 corresponds to pH 5 (acidic), not neutral.

Common Pitfalls:
Forgetting that Kw changes with temperature; at 25°C it is 1e-14.

Final Answer:
1e-7 mol/L