[Ilikebugs]

Methanoic acid, HCOOH, is a weak electrolyte. In a
solution prepared by dissolving 0.10 mol HCOOH in
water to make 1.0 L of solution, approximately 4.1% of
the HCOOH molecules ionize. What is the pH of this
solution?

I know that the pH is -log(molarity of H+ ions), but I don't know how to calculate the molarity. I got that 0.0041 moles of molecules ionize but that's only the molarity of HCOOH.

[Borek]

You know number of moles, you know the volume, just plug these numbers into molarity definition.

[Ilikebugs]

But isn't that the molarity of HCOOH ions, not H ions?

[AWK]

HCOOH is a neutral molecule

[Borek]

I got that 0.0041 moles of molecules ionize but that's only the molarity of HCOOH.

Actually on the second read I see you are confused, but I am not sure about what.

You are told 4.1% of the HCOOH dissociated. You have correctly calculated concentration of the HCOOH that dissociated. Just from the stoichiometry of dissociation (write the reaction equation) you can easily calculate concentration of the produced H⁺. No idea at which step you get lost.