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D ETERMINATION OF THE K A OF A W EAK A CID Introduction : Acids and bases are often described as being “weak” or “strong”. While this classification seems somewhat arbitrary, other more quantitative descriptors exist. For acids, Ka values are commonly used. The ionization of an acid can be shown by the following equation: HA (aq) H + (aq) + A - (aq) (Eq. 1) Since an equilibrium exists, an equilibrium constant, Ka, can be written: K a = [H + ] [A - ] (Eq. 2) [HA] The K a value is an indication of acid strength. The larger the value of the Ka, the stronger the acid. This value is characteristic of the acid and can be used to help identify an unknown acid. A similar system exists for bases (K b ). Two methods may be used to determine the K a value. Both methods require the use of a pH meter. In the first method, a sample of acid is titrated with base. The pH values are plotted vs. the volume of base added. The equivalence point is determined from the graph. Next, the volume of base halfway to the equivalence point is found, and the pH at this volume is noted. The [H + ] corresponding to this pH is equal to the K a for the acid. At a point halfway to the equivalence point, [H + ] = [HA] = [A - ] for a monoprotic acid. Canceling out [A - ] and [HA] in Equation 2 gives K a = [H + ]. The second method for determining K a values involves a “half volume” method. A solution of the acid is prepared and divided in half as accurately as possible. One portion is titrated to its endpoint with phenolphthalein. The two portions are then recombined, and the pH of the resulting solution is measured. Since half of the acid has been titrated, [H + ] = [HA] = [A
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This note was uploaded on 04/20/2008 for the course CHEM 1310 taught by Professor Cox during the Fall '08 term at Georgia Tech.

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