A pH of 10 or 11 seems reasonable. Using Eq. H ]\!\!\text{ }} \label{3}\], Under most circumstances we can make the approximation, in which case Equation \(\ref{3}\) reduces to the approximation. We can assume that x is so small that it will be meaningless by the time we use significant figures. To find the pH we follow the same general procedure as in the case of a weak acid. H However, pH of bases is usually calculated in terms of the OH− concentration. The formula for pH is: Legal. As shown above, the pH of the solution, which depends on the H+ concentration, increases with increasing OH− concentration; a greater OH− concentration means a smaller H+ concentration, therefore a greater pH. An example of a weak base is ammonia. [\!\!\text{ OH}^{-}\text{ }\!\! H w [\!\!\text{ B }\!\! A weak base is less protonated, leading to a lower pH and a lower percentage protonated.[2]. A weak base is a base that, upon dissolution in water, does not dissociate completely, so that the resulting aqueous solution contains only a small proportion of hydroxide ions and the concerned basic radical, and a large proportion of undissociated molecules of the base. − which is identical to the expression obtained in the acid case (approximation shown in equation 6 in the section on the pH of weak acids) except that OH– replaces H3O+ and b replaces a. A weak base will have a higher H+ concentration than a stronger base because it is less completely protonated than a stronger base and, therefore, more hydrogen ions remain in its solution. ]\!\!\text{ )}}\) (5). A pH of 13 or 14 would be too basic, while a pH of 8 or 9 is too close to neutral. Have questions or comments? K = 2) Ammonia (formula = NH 3) is the most common weak base example used by instructors. While strong bases release hydroxide ions via dissociation, weak bases generate hydroxide ions by reacting with water. ex. Bases increase pH; weak bases have a less dramatic effect on pH. ]\!\!\text{ }} \label{2}\]. Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn. This table is part of our larger collection of acid-base resources. p (14.5.1) B + H 2 O ⇌ BH + + OH −. ]\!\!\text{ }^{\text{2}}}{c_{b}-\text{ }\!\! Strong bases have smaller H+ concentrations because they are more fully protonated, leaving fewer hydrogen ions in the solution. Instead of an acid constant Ka, a base constant Kb must be used. From the equation for percentage protonated with [HB. ] A list of Kb values for selected bases arranged in order of strength is given in the table below. If a weak base B accepts protons from water according to the equation, \[\text{B} + \text{ H}_{\text{2}}\text{O}\rightleftharpoons\text{BH}^{+} + \text{OH}^{-} \label{1}\], then the base constant is defined by the expression, \[K_{b}=\dfrac{\text{ }\!\! K O Guide to Weak Bases from Georgetown course notes, Article on Acidity of Solutions of Weak Bases, https://en.wikipedia.org/w/index.php?title=Weak_base&oldid=961170760, Creative Commons Attribution-ShareAlike License, Substitute the equilibrium molarities into the basicity constant. O 1.8 x … A weak base persists in chemical equilibrium in much the same way as a weak acid does, with a base dissociation constant (Kb) indicating the strength of the base. The appropriate formula can be derived from Eq. [ 5) weak bases acquire hydrogen ions from water leaving hydroxide . Using the value for Kb listed in the table, find the pH of 0.100 M NH3. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the solution is said to have a pH greater than 7.0 at standard conditions, potentially as high as 14 (and even greater than 14 for some bases). Basic Information . − A lower percentage will correspond with a lower pH because both numbers result from the amount of protonation. p The pH of a weak base falls somewhere between 7 and 10. Given its greater H+ concentration, the formula yields a lower pH value for the weak base. Once we have found the hydroxide-ion concentration from this approximation, we can then easily find the pOH, and from it the pH. Watch the recordings here on Youtube! O [\!\!\text{ OH}^{-}\text{ }\!\! Clark, Jim. = [ The formula for pH is: Bases are proton acceptors; a base will receive a hydrogen ion from water, H2O, and the remaining H+ concentration in the solution determines pH. Freeman, 2005. Chemical Principles: The Quest for Insight, 3rd Ed., New York: W.H. Web. The approximation is valid, and we thus proceed to find the pOH. K [ "article:topic", "pH", "pH of Solutions of Weak Bases", "weak base", "authorname:chemprime", "showtoc:no", "license:ccbyncsa" ], Ed Vitz, John W. Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, & Adam Hahn, Chemical Education Digital Library (ChemEd DL), equation 4 in the section on the pH of weak acids, equation 6 in the section on the pH of weak acids, \(NH_3 + H_2O \rightleftharpoons NH^+_4 + OH^–\), \(C_6H_5NH_2 + H_2O \rightleftharpoons C_6H_5NH^+_3 + OH^–\), \(CO_3^{2–} + H_2O \rightleftharpoons HCO^-_3 + OH^–\), \(N_2H_4 + H_2O \rightleftharpoons N_2H^+_5 + OH^–\), \(PO_4^{3–} + H_2O \rightleftharpoons HPO^{2-}_4 + OH^–\), \(C_5H_5N + H_2O \rightleftharpoons C_5H_5NH^+ + OH^–\), Taken from Hogfelt, E. Perrin, D. D. Stability Constants of Metal Ion Complexes, 1. Like weak acids, weak bases do not undergo complete dissociation; instead, their ionization is a two-way reaction with a definite equilibrium point. p Bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the solution is said to have a pH greater than 7.0 at standard conditions, potentially as high as 14 (and even greater than 14 for some bases). The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Since we have a dilute solution of a weak base, we expect the solution to be only mildly basic. To help describe the strengths of weak bases, it is helpful to know the percentage protonated-the percentage of base molecules that have been protonated. This calculated value checks well with our initial guess. 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