Therefore, the concentration of H+ ions in a weak acid solution is always less than the concentration of the undissociated species, HA. The generalized dissociation reaction is given by: [latex]HA(aq) \rightleftharpoons H^+ (aq) + A^- (aq)[/latex]. Solution Acetic acid (CH3COOH) and oxalic acid (H2C2O4) are examples of weak acids. Sulfuric acid is considered strong only in its first dissociation step; 100 percent dissociation isn't true as solutions become more concentrated. Organic acids containing one or more carboxyl group, -COOH. CC BY-SA 3.0. https://en.wikipedia.org/wiki/Weak_acid%23Weak_acids The H is ionizable. The equilibrium principles are essential for the understanding of equilibria of weak acids and weak bases. A molecule with a formula starting with H usually is an acid. \ce{HA &\rightleftharpoons &H+ &+ &A- &}\\ The conjugate base of a weak acid is a weak base, while the conjugate acid of a weak base is a weak acid. The first Ka refers to the first dissociation step: [latex]H_2CO_3 + H_2O \rightarrow HCO_3^{-} + H_3O^+[/latex]. List of weak electrolyte examples can generally be formed from weak acids or bases as well. Wiktionary Weak acids. For example, H2CO3 has two Ka values because it has two acidic protons. Ethanoic acid is a typical weak acid. In this case, the discussion is similar and parallel to that given above for the calculation of pH of weak acids when Ka is known. Acids with a Ka less than 1.8×10−16 are weaker acids than water. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Petrucci R.H., Harwood, R.S. The pKa of acetic acid is 4.75. Note that Ka = 10-pKa. Acetic acid, \(\ce{CH3COOH}\), is a typical weak acid, and it is the ingredient of vinegar. \[K = \dfrac{(0.0042)^2}{1.000} = 1.78\times10^{-5}\], The equilibrium constant of an acid is represented by Ka; and similar to the pH scale, a pKa scale is defined by, \[\mathrm{p\mathit K_a = - \log \mathit K_a}\], and for acetic acid, pKa = 4.75. If an acid is represented by HA, then there is a balance between molecules which are split (ionised), and molecules which are not: The relative strength of an acid can be expressed using Ka, the acid ionization constant. and see what you get. Stronger acids have a larger acid dissociation constant (Ka) and a smaller logarithmic constant (pKa = −log Ka) than weaker acids. Most weak bases are anions of weak acids. The Ka of acetic acid is [latex]1.8\times 10^{-5}[/latex]. Wikibooks Practice. Strong acids completely dissociate in water, forming H+ and an anion. \[x = \dfrac{- K_{\ce a} + \sqrt{K_{\ce a}^2 + 4 C K_{\ce a}}}{2}\]. \end{align}\). She has taught science courses at the high school, college, and graduate levels. The strength of a weak acid is represented as either an equilibrium constant or a percent dissociation. Weak electrolytes are incompletely dissociated. It is partially ionized in its solution. At equilibrium, the weak acid, its conjugate base, and the hydrogen ion are all present in the aqueous solution. Weak acids and bases are only partially ionized in their solutions, whereas strong acids and bases are completely ionized when dissolved in water. Then its ionization can be written as: In a solution whose label concentration is \(C = \ce{[HA]} + \ce{[A- ]}\), let us assume that x is the concentration that has undergone ionization. Wikipedia Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (January 2005). There are six strong acids. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. A common assumption is that x is small; we can justify assuming this for calculations involving weak acids and bases, because we know that these compounds only dissociate to a very small extent. For example, H 2 CO 3 has two K a values because it has two acidic protons. Thus, \(\mathrm{pH = -\log0.0042 = 2.376}\). Strong acids are listed at the top left hand corner of the table and have Ka values >1 2. From Simple English Wikipedia, the free encyclopedia, https://simple.wikipedia.org/w/index.php?title=Weak_acid&oldid=6368679, Creative Commons Attribution/Share-Alike License. conjugate acidthe species created when a base accepts a proton. Weak Acids HO 2 C 2 O 2 H - oxalic acid H 2 SO 3 - sulfurous acid HSO 4 - - hydrogen sulfate ion H 3 PO 4 - phosphoric acid HNO 2 - nitrous acid HF - hydrofluoric acid HCO 2 H - methanoic acid C 6 H 5 COOH - benzoic acid CH 3 COOH - acetic acid HCOOH - formic acid