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Weak base
In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As BronstedLowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH level compared to strong bases. Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14). The pH level has the formula: Since bases are proton acceptors, the base receives a hydrogen ion from water, H_{2}O, and the remaining H^{+} concentration in the solution determines the pH level. Weak bases will have a higher H^{+} concentration because they are less completely protonated than stronger bases and, therefore, more hydrogen ions remain in the solution. If you plug in a higher H^{+} concentration into the formula, a low pH level results. However, the pH level of bases is usually calculated using the OH^{} concentration to find the pOH level first. This is done because the H^{+} concentration is not a part of the reaction, while the OH^{} concentration is. By multiplying a conjugate acid (such as NH_{4}^{+}) and a conjugate base (such as NH_{3}) the following is given: Since K_{w} = [H_{3}O^{ + }][OH^{ − }] then, By taking logarithms of both sides of the equation, the following is reached:
Finally, multipying throughout the equation by 1, the equation turns into:
After acquiring pOH from the previous pOH formula, pH can be calculated using the formula pH = pK_{w}  pOH where pK_{w} = 14.00. Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a Base Ionization Constant (K_{b}) (or the Base Dissociation Constant) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up: Bases that have a large K_{b} will ionize more completely and are thus stronger bases. As stated above, the pH of the solution depends on the H^{+} concentration, which is related to the OH^{} concentration by the Ionic Constant of water (K_{w} = 1.0x10^{14}) (See article Selfionization of water.) A strong base has a lower H^{+} concentration because they are fully protonated and less hydrogen ions remain in the solution. A lower H^{+} concentration also means a higher OH^{} concentration and therefore, a larger K_{b}.
Additional recommended knowledge
Percentage protonatedAs seen above, the strength of a base depends primarily on the pH level. To help describe the strengths of weak bases, it is helpful to know the percentage protonatedthe percentage of base molecules that have been protonated. A lower percentage will correspond with a lower pH level because both numbers result from the amount of protonation. A weak base is less protonated, leading to a lower pH and a lower percentage protonated. The typical proton transfer equilibrium appears as such: B represents the base. In this formula, [B]_{initial} is the initial molar concentration of the base, assuming that no protonation has occurred. A typical pH problemCalculate the pH and percentage protonation of a .20 M aqueous solution of pyridine, C_{5}H_{5}N. The K_{b} for C_{5}H_{5}N is 1.8 x 10^{9}. First, write the proton transfer equilibrium: The equilibrium table, with all concentrations in moles per liter, is
This means .0095% of the pyridine is in the protonated form of C_{5}H_{6}N^{+}. Examples
Other weak bases are essentially any bases not on the list of strong bases. See also
References


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Weak_base". A list of authors is available in Wikipedia. 