Lab 3- Determination of the Dissociation Constant of a Weak Acid.docx

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CHM 106Lab 3: Determination of the Dissociation Constant of a Weak AcidDetermination of Kafor a weak acid in water:A weak acid, HA, dissociates into H+ions and the conjugate base, A-ions,as shown in thefollowing equilibrium equation:HA ↔ H++ A-Equation (1)The equilibrium constant, sometimes called ionization constant ordissociationconstant, is:Ka=[H+][A][HA]Equation(2)If we have only the acid in water, then equal amounts of H+and A-are formed, so:[H+] = [A-]Equation (3)If we can determine the concentration of [H+] and the concentration of [HA], wecan calculate Ka. In this experiment, we will titrate the acid with a standard NaOHsolution to determine [HA], and we will measure the pH of the original acidsolution to determine [H+].Determination of Kafrom a buffer solution:As NaOH is added in a titration, the concentrations of [H+] and [HA] will go down,and the concentration of [A-] will go up. Thus Kais the product of [H+] multipliedby the ratio of [A-]/[HA]. Again, we will determine the concentration of [H+] bymeasuring the pH. We can adjust the ratio of [A-]/[HA] by varying the amount ofNaOH added. In the experiment, we will determine Kathree times by varying theratio.Abuffersolution exists at any point during the titration, except for the original acidand the equivalence point and beyond. All the points between the beginning and endpoint have appreciable mounts of both weak acid and conjugate base, and such asolution is called a buffer solution.

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Term
Spring
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Tags
pH, kA

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