Chemistry (including Biochemistry)/pH


I have made vinegar which registers at a pH of 3 on a test strip. I would like to know how much more acidic (what percentage) is this than a vinegar with an acidity of 5%, or what is the pH of a 5% vinegar solution?  Thank you. DAWN

First and easiest method would be to make a 5% vinegar solution.

Failing that you can calculate this pretty easily, but you will need to know something about molarity, molecular weight, concentration and weak acids (which you should read up on at wikipedia before proceeding) - pH is direct result of these  and means nothing without context.

Since acetic acid has a density of about 1.049 g/mL, let's imagine a solution that is 5 parts acetic acid, and 95 parts water.  Lets scale it to a liter, we then have 950 mL of water and 50 mL of pure acetic acid.  When you scale this to the 1L of water (which you will see is an important consideration for molarity) you get 52.63 mL of acetic acid - using the density you now have 55.2 grams of acetic acid.

The molecular weight for acetic acid is 60.05 g/mol - so this means that our concentration is 0.92 M.  However, we cannot immediately call this the concentration protons (protons being what makes it an acid). Because acetic acid does does not fully dissociate, it has a pKa of 1.8 x 10^5.  This means 1.5 x 10^-5 = [H+]^2/[HA] and a decent approximation is that the H+ change does not effect HA. So we then solve based on our known HA = 0.92.

sqrt( 1.8 x 10^-5 * 0.92) = [H+] = 0.003715 moles/gram

The final step is to see how many moles of [H+] are in your pH 3 vinegar.

The fundamental equation for determining an acid solution pH is

pH = -log[H+]

for your vinegar

3 = -log [H+]

this means [H+] = 0.001

Lastly, this means your vinegar is 0.001 vs 0.0037 for a 5% acetic acid solution.  So the 5% solution is about 3.7 times more acidic than your vinegar.

This also means that your vinegar is about equivalent to 1.35% vinegar.  

Another way to talk about it would be to say that the 5% solution is pH 2.43.

I hope this helps!

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Dr. Jeffery Raymond


Materials chemistry. Materials science. Spectroscopy. Polymer science. Physical Chemistry. General Physics. Technical writing. General Applied Mathematics. Nanomaterials. Optoelectronic Behavior. Science Policy.


Teaching: General Inorganic Chemistry I & II, Organic Chemistry I & II, Physical Chemistry I, Polymeric Materials, General Physics I, Calculus I & II
My prior experience includes the United States Army and three years as a development chemist in industry. Currently I am the Assistant Director of the Laboratory for Synthetic Biological Interactions. All told, 13 years of experience in research, development and science education.

Texas A&M University, American Chemical Society, POLY-ACS, SPIE

Journal of the American Chemical Society, Nanoletters, Journal of Physical Chemistry C, Journal of Physical Chemistry Letters, Ultramicroscopy Proceedings of SPIE, Proceedings of MRS, Polymer News, Chemical and Engineering News, Nano Letters, Small,, Angewandte

PhD Macromolecular Science and Engineering (Photophysics/Nanomaterials Concentration), MS Materials Science, BS Chemistry and Physics, Graduate Certificate in Science Policy, AAS Chemical Technology, AAS Engineering Technology

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