# 2046 Lab 06 Determination of the Ionization Constant of a Weak Acid.pdf

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Lab 06Determination of the Ionization Constant of a Weak AcidIntroductionIn this experiment you will determine an approximate value for the ionization constant,Ka, of an unknown organic monoprotic weak acid.The equation for the ionization of theweak acid is:HAH++ Aand the Kais the equilibrium constant for the process:Ka=[H+][A][HA](1)By solving this relation for [H+] one obtains the equation[H+] =[HA] Ka[A](2)A weak acid solution is so slightly ionized that the concentration of its undissociatedmolecules is very nearly equal to its total concentration.However, a salt of such an acidis so highly ionized in solution that the concentration of its anions is very nearly equal toits total concentration.If one partially neutralizes a weak monoprotic acid with one halfof the base needed for complete neutralization, the concentration of salt formed isessentially equal to the concentration of the remaining acid, and the ratio [HA] / [A] ofEquation 2 can be considered to be equal to 1.Thus [H+] = Kaand -log [H+] = pKa.Bydefinition, pH = -log Ka.It follows, then, under these conditions of half-neutralization wefind thatpH = pKa(3)The measurement you will make takes advantage of this reasoning for bufferedsolutions.You will estimate the pH of a half-neutralized solution of a weak acid byadding an acid-base indicator (methyl orange or bromcresol green), and comparing thecolor with the color of standard HCl acid solutions of known pH containing the sameindicators.The estimated pKaof the weak acid is equal to the pH value of the matchingstandard color, and from this the value of Kacan be calculated.Pre Lab Problems (answer on separate paper)1) 10.00 mL of a solution prepared by combining 0.0022 mol of a weak acid and 0.0011mol NaOH was found to have a pH of 4.2.What is the pKavalue for this acid?TheKavalue?
Lab 062) A student carried out this experiment with a weak acid having pKa= 3.40.Bymistake, 11.00 mL of 0.100M NaOH were added to 10.00 mL of 0.200M acid (10%too much NaOH).What would the pH have been if the student had correctly addedonly 10.00 mL of the NaOH solution?What was the pH of the mixture as it wasactually prepared?EquipmentHCl approximately 0.010M14 – 18 x 150 mm Test tubesNaOH approximately 0.10MDistilled waterUnknown weak acid approximately 0.2MTest tube rackPhenolphthalein 1% in ethanol125 mL Erlenmeyer flask(s)Methyl Orange 0.1% in water10 mL Graduated cylinderBromcresol Green sodium salt 0.1% in water50 mL Graduated cylinderBuret(s)Procedure

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