Why does pka equal ph

Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.

Facebook Facebook Twitter Twitter. Updated January 30, The lower the pKa, the stronger the acid and the greater the ability to donate a proton in aqueous solution.

The Henderson-Hasselbalch equation relates pKa and pH. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. To be honest, I just don't get it. I had this question while reading a Chemguide article, on this topic:. I think my confusion is caused by the way I look at pKa of acid-base indicator. Should you think of it more as a reaction quotient. Because technically, at equilibrium, the two concentration aren't the same so cannot be cancelled out.

So the idea is that the indicator should indicate when the reaction is complete, not the equivalence point of the reaction.

It does not. The equivalence point depends on the reaction you are monitoring with the indicator. So you would first figure out what the pH at the equivalence point is. Then you would choose an indicator that changes color near that pH. The pH indicator reaction reaches equilibrium after less than a couple of seconds. If you are asked to say something about the basicity of ammonia NH 3 compared to that of ethoxide ion CH 3 CH 2 O - , for example, the relevant pK a values to consider are 9.

From these numbers, you know that ethoxide is the stronger base. Do not make the mistake of using the pK a value of this is the pK a of ammonia acting as an acid , and tells you how basic the NH 2 - ion is very basic! Biochemistry and organic chemistry texts often list the value as These texts have incorrectly factored the molar value for the concentration of water into the equilibrium constant.

The correct derivation of the equilibrium constant involves the activity of water, which has a value of 1. While this course begins with single functional groups, we will eventually work with interesting compounds containing multiple functional groups.

Recognizing which hydrogens can be ionized as acidic protons and which hydrogens can NOT, is a useful skill. For example, the pKa value of lactic acid is about 3.

Another important point is the relationship between pH and the pKa of an acid. This relationship is described by the following equation. If the pH changes by 1 near the pKa value, the dissociation status of the acid changes by an extremely large amount. In the case of acetic acid, for example, if the solution's pH changes near 4.

When the pH is 3.