Which is More Acidic: Water or Hydrogen Fluoride?

Q: Which is more acidic, Water or Hydrogen Fluoride?

Hydrogen Fluoride (pKa 3.2) is more acidic than Water (pKa 15.7). A lower pKa indicates a stronger acid.

Which is more acidic, Water or Hydrogen Fluoride?

Hydrogen fluoride (pKa 3.2) is about 12 orders of magnitude more acidic than water (pKa 15.7). Fluorine is the most electronegative element (EN = 4.0 vs 3.5 for oxygen), so F- handles a negative charge better than OH-. Hydrogen Fluoride has a pKa of 3.2, while Water has a pKa of 15.7.

Molecule A	Water (H₂O), pKa 15.7
Molecule B	Hydrogen Fluoride (HF), pKa 3.2
More Acidic	Hydrogen Fluoride
Governing Factor	Electronegativity
Difficulty	Intermediate

Introduction

Water and HF are both small molecules with a hydrogen bonded to a highly electronegative atom. Which one releases its proton more easily?

Acidic Protons

Water has two O-H bonds, and HF has one H-F bond. In each case, the highlighted atom is where the negative charge ends up after the proton leaves.

Governing Factor: Electronegativity

The key factor is electronegativity. Fluorine (EN = 4.0) is the most electronegative element on the periodic table. Oxygen (EN = 3.5) is electronegative too, but fluorine wins. The more electronegative atom stabilizes the conjugate base better.

Conjugate Base Stability

Hydroxide (OH-) has its charge on oxygen (EN 3.5). Fluoride (F-) has its charge on fluorine (EN 4.0). Fluoride holds the extra electrons more tightly, making it a weaker base and HF the stronger acid.

pKa Comparison

Water has a pKa of 15.7 and HF has a pKa of 3.2. The 12.5-unit difference corresponds to HF being roughly 10^12 times more acidic. This is purely an electronegativity effect within the same period.

Interactive side-by-side 3D viewer with acidic proton highlights, conjugate base overlays, and pKa labels.

Compare Water and Hydrogen Fluoride in 3D