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libby's part

# libby's part - Experiment 28 A SelfDirected Experiment...

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Experiment 28: A Self- Directed Experiment Identifying an Unknown Weak Acid Martin Steine Courtney V. Lyons Elizabeth Cordell December 1, 2008

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Purpose of the experiment: Determine the acid ionization constant, (K a ), of an unknown weak acid through three different experimental processes. Background Information: Acid strength of a substance is vital in several situations. For instance, the acidity of a substance can affect the quality of drinking water, soil conditions, and food preservation. Therefore, it is exceptionally important to be able to determine the acid ionization constant, (K a ), of a substance. The acid ionization constant is a specific equilibrium constant that measures the strength of an acid. There is a direct relationship between the acid ionization constant and acid strength. Thus, as the K a value increases the acid becomes stronger and as the K a value decreases the acid becomes weaker. When a generic acid dissolves in water, it dissociates forming a hydronium ion and the conjugate base of the weak acid. The following equation illustrates the ionization of a generic acid: HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq). The acid ionization constant can be defined and expressed by the following equation: . In this experiment, three different experimental methods will be used in order to discover and identify the K a value of an unknown weak acid. The three experimental methods are: 1) titrating the acid with 0.100 M NaOH; 2) determining the percent ionization of the acid through freezing point depression; and 3) determining the pH of the acid. The first experimental procedure, titration of an acid with 0.100 M NaOH, will produce a titration curve which plots the pH of the acid solution vs. the volume of added NaOH. From the titration curve, the equivalence point and the half equivalence point can be determined. The equivalence point is the point at which all the weak acid has
completely reacted with NaOH and is located on the graph at the center of the sharp increase of pH. The half equivalence point is the point at which exactly one half of the base required to completely neutralize the acid has been added. Due to the fact that the concentration of the acid in the solution and the concentration of its conjugate base are equal at the half equivalence point, the following equation can be derived: K a = [H 3 O + ]. Then by taking the negative logarithm of each side of this equation, the following equation can be derived: pK a = pH. Finally, the K a of the acid is determined from the following equation: K a = 10^(-pK a ). The second experimental method, determination of pH of the acid, also can determine the K a of an unknown acid. However, in this method the pH and the initial weak acid concentration in the solution must be known. If the pH of the acid solution is known, the H + and A concentrations at equilibrium can be calculated. The K a value can be determined by substituting the concentrations at equilibrium into the following equation: [H 3 O + ] = 10^(-pH). Lastly, the K a value can be determined through the method of percent ionization of the acid from freezing point depression. The freezing point

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