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Unformatted text preview: Weak Acids Titration Author: Steven Hagedon Lab Partners: Danielle Nguyen, Ashley Cox, And Grishma Sheth Instructor: Yan Zhao Chemistry 104B, Section 027 Date Work Performed: January 12, 2008 and January 19, 2008 Date Work Submitted: January 26, 2008 Abstract The purpose of this experiment was to determine the identity of an unknown acid, which was in the solid form. This was done, by first, standardizing a NaOH solution, whose average molarity was .1191 M, to be used in a titration with a KHP solution, then being titrated, the next week, into a solution containing distilled water and the unknown acid. The molarities of the three KHP solutions are: .1156 M, .1241 M, and . 1176 M. These molarities created a standard deviation of .004444. The identification of the unknown solid was found through the use of molecular weight, of which was experimented to be 54.1 g/mol, which is similar to that of Glycolic acid, whose molecular weight is 79.05 g/mol. Therefore, the unknown acid is glycolic acid in the solid form, with 99% purity. The mass of the glycolic acid that was used was .226 grams, while the moles of NaOH are equal to the moles of the acid, which was experiment to be .004026 moles. The experimented pK a is 4.07 and the K a is 8.85x10-5 . The equivalence point, from the titration curve, took place at 33.8 mL and pH 7.9, resulting in the half-equivalence point resulting at 16.9 mL and pH 4.07. Introduction The purpose of the experiment was to determine the identity of an unknown acid, based upon its properties of K a value, molecular weight, pH, and pK a . The K a is known as the weak acid dissociation constant and is determined by the equilibrium expression of K a = [H 3 O + ] eq [A- ] eq /[HA] eq . The use of a titration is needed to produce a titration curve, where the equivalence point and the half-equivalence point were used to determine the pK a value. The half-equivalence point is located within the buffer region of a titration curve, the area leading to the neutralization point of the solution, also known as the equivalence point, where the number of moles of base is equal to the number of moles of acid. The pH at the equivalence point should be in the neutral region, around pH 7.0 to pH 7.9. The pH is found by the equation: pH = pKneutral region, around pH 7....
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