Confused about the pH of battery acid

[baniskher]

I'm not a chemist nor a chemistry student.

I'm trying to understand why the pH of battery acid doesn't match what I expect it to be.

My understanding is that each molecule of sulfuric acid should donate two free hydrogen atoms per molecule of H ₂ SO ₄ .

If I look up the molarity of battery acid, I find that it is about 4.2–5.0 mol/L.

Water doesn't contribute enough hydrogen to worry about, so I should be able to ignore that contribution.

That means there should be about 9 moles of free hydrogen per liter of battery acid.

The formula for pH is -log ₁ ₀ (moles free H⁺/liter), so I would expect a pH of about -log₁ ₀ (9)= -0.95.

But the pH of battery acid is listed as 0.8, not -0.9.

What am I not getting here?

[YogaChem]

Only the first hydrogen in sulfuric acid fully dissociates.  Bisulphate (the conjugate to sulfuric acid) provides the second hydrogen atom, but is a very weak acid, so will not dissociate in a battery in any meaningful way when compared to the first hydrogen atom.  Therefore, in a calculation of this type, you can safely assume that a 4.2 mol/L solution is only account for one hydrogen atom, not two.  Hope that helps