The pKa of sulfuric acid is -10, and of hydrochloric acid is -7. We use cookies to help provide and enhance our service and tailor content and ads. In fact, for an dilute aqueous solution, the activity of water is approximated with the value of 1, so the generic dissociation constant for a given acid HA or HB+ is expressed as: The value of Ka for acetic acid is 1.75 x 10-5 - much less than 1, indicating there is much more acetic acid in solution at equilibrium than acetate and hydronium ions. In some embodiments the acid is phosphoric acid, sulfuric acid, hydrochloric acid, boric acid, nitric acid, or a carboxylic acid such as acetic acid. PageIndex: ["{3.6. In what state is the side chain functional group: the protonated state (a carboxylic acid) or the deprotonated state (a carboxylate ion)? Download : Download high-res image (167KB)Download : Download full-size image. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. For more information contact us at or check out our status page at What is the ratio of acetate ion to neutral acetic acid when a small amount of acetic acid (pKa = 4.8) is dissolved in a buffer of pH 2.8? Group work Draw each molecule. Explain your answer. When acetic acid is reacted with ammonia,ammonium acetate is obtained. On the left side, the acid is of course acetic acid while on the right side the acid is methyl ammonium ion (in other words, methyl ammonium ion is the acid in the reaction going from right to left). A number like 1.75 x 10- 5 is not very easy either to say, remember, or visualize, so chemists usually use a more convenient term to express relative acidity. The Henderson-Hasselbalch equation is particularly useful when we want to think about the protonation state of different biomolecule functional groups in a pH 7 buffer. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. where ΔpKa = (pKa of product acid minus pKa of reactant acid). (The actual composition of physiological buffer is complex, but it is primarily based on phosphoric and carbonic acids). Download : Download high-res image (57KB)Download : Download full-size image. The value of K a for acetic acid is 1.75 x 10-5 - much less than 1, indicating there is much more acetic acid in solution at equilibrium than acetate and hydronium ions. TeX: { This is also consistent with the fact that no significant racemization/epimerization were observed in the amide products when asymmetric esters and/or amines were used as the reactants, because concerted acyl substitution mechanism precludes the intermediacy of tetrahedral adducts and the accompanying generation/elimination of new chiral centers. Would you expect phenol to be soluble in an aqueous solution buffered to pH 2? The amine group is also not acidic, its pKa is about 35. The most important of these are summarized below, with very rough pKa values for the conjugate acid forms. *alpha-carbon acids will be discussed later in this chapter. Conversely, sulfuric acid, with a K a of approximately 10 9 , or hydrochloric acid, with a K a of approximately 10 7 , both undergo essentially complete dissociation in water: they are very strong acids. } (Remember, uncharged amines are basic: it is positively-charged protonated amines, with pKa values around 10, that are weakly acidic.) Now, let's use the equation again, this time for an amine functional group, such as the side chain of a lysine residue: inside a cell, are we likely to see a neutral amine (R-NH2) or an ammonium cation (R-NH3+?) Doing the math, we see that, Keq = 10ΔpKa = 10(10.6 – 4.8) = 105.8 = 6.3 x 105. pH 12? If this molecule were to react with one molar equivalent of a strong base such as sodium hydroxide, it is the phenol proton which would be donated to form a phenolate anion. This result extends to all other carboxylic acid groups you might find on natural biomolecules or drug molecules: in the physiological environment, carboxylic acids are almost completely deprotonated. When we do this, we are always assuming that the concentration of the biomolecule is small compared to the concentration of the buffer components. Which is the stronger base, CH3O- or CH3S-? It is important to realize that pKa is not the same thing as pH: the former is an inherent property of a compound or functional group, while the latter is a measure of hydronium ion concentration in a given aqueous solution: Knowing pKa values not only allows us to compare acid strength, it also allows us to compare base strength.