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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Ilikebugs on April 26, 2019, 11:17:59 PM

Title: Finding the pH given an amount of diluted Methanoic acid.
Post by: Ilikebugs on April 26, 2019, 11:17:59 PM

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.

Title: Re: Finding the pH given an amount of diluted Methanoic acid.
Post by: Borek on April 27, 2019, 03:10:26 AM

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

Title: Re: Finding the pH given an amount of diluted Methanoic acid.
Post by: Ilikebugs on April 27, 2019, 05:47:01 PM

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

Title: Re: Finding the pH given an amount of diluted Methanoic acid.
Post by: AWK on April 27, 2019, 06:12:20 PM

HCOOH is a neutral molecule

Title: Re: Finding the pH given an amount of diluted Methanoic acid.
Post by: Borek on April 27, 2019, 06:54:13 PM

Quote from: Ilikebugs on April 26, 2019, 11:17:59 PM

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.