At half the equivalence point:. It's worth noting sometimes this equation is written for the K a value rather than pKa, so you should know the relationship:. The reason the Henderson-Hasselbalch equation is an approximation is because it takes water chemistry out of the equation. You shouldn't try to apply the approximation for concentrated solutions. Use the approximation only when the following conditions are met:. The K a value from a table of HNO 2 is 5.
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Select personalised ads. Apply market research to generate audience insights. Measure content performance. An application of the Henderson-Hasselbach Equation is the ability to determine the relative acidity of compounds by comparing their pKa values. Some selected pKa values for compounds in the study of organic chemistry are shown bellow. Since organic reactions can be performed in non-aqueous environments, the pH can exceed 14 and organic compounds can have pKa values above It is a variation on that line from the Wizard of Oz, "We don't live in water anymore.
It is a very good idea to commit to memory the approximate pK a ranges of the compounds above. 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. That "signature" trait is called the acid dissociation constant K a. Sometimes informally written as ka, you can calculate pH in a mathematically straightforward manner. An acid is a molecule that can donate a proton and rarely, more than one proton in sequence in aqueous solution, i.
The molecule left behind is an anion. Strong acids such as hydrochloric acid HCl more "eagerly" donate protons than the far more numerous weak acids, meaning that they can offload protons even in a low-pH environment, i. Weak acids are only eager to donate their protons when the ambient pH is high, that is, the proton concentration is relatively low.
Above, you read that a low pH implies an environment with lots of protons freed of their parent acids. As it happens, the pH scale is a logarithmic or "log" scale that for practical purposes ranges from 1 to 14, from most to least acidic.
The equation for pH is:. Every tenfold increase in proton concentration drives the pH down by one integer unit and conversely. K a thus offers a measure of an acid's "enthusiasm" to offload protons and thus is strength; the more strongly dissociated the acid at equilibrium, the higher the numerator in relation to the denominator in this equation and the higher the K a.
You can calculate the pH of a solution given the pKa of the acid and the concentrations above, that of the donated protons excluded.
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