Exp 6 Determination of the Ionization Constant of a Weak Acid-2.docx

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CHEM-102Experiment #6Determination of the Ionization Constant of a Weak AcidPre-Lab Activities:After reading the lab, complete items a, b, c, and d (title, purpose, chemicals andequipment, and summary of procedure) as described on page10 of Exp. 1 on an8 1/2 x 11 sheet of paper.Bring a USB drive to store your lab dataAnswer the following questions on 8½ x 11 sheet of paper or in your laboratorynotebook if one is required by your instructor:1.What are the largest sources of error in this experiment?2.If 30.15 mL of 0.0995 M NaOH is required to neutralize 0.180 g of anunknown acid, HA, what is the molecular weight of the unknown acid?Lab Activities:Go over the prelab questions with your lab instructorComplete lab and fill in data sheet for the Standardization of NaOH (part A).Parts B-D will be completed the following lab period.Objective:To experimentally determine the ionization constant (Ka) and molecular weight(MW) of an unknown weak acidTo identify the unknown acidIntroduction:NaOH stock solution is standardized by titration against primary standard potassiumhydrogen phthalate, KHC8H4O4. The concentration and Kaof the unknown acid is thendetermined by titrating the acid against the standardized NaOH using a pH probe and acomputer program. The unknown acid may be identified from a list of possibilities usingboth the Kaand molecular weight found experimentally.1 |P a g e
According to the Bronsted-Lowry acid-base theory, the strength of an acid is related toits ability to donate protons.All acid-base reactions are then competitions betweenbases of various strengths for these protons.For example, the strong acid HCl reactswith water according to Equation (1):HCl+H2OH3O++Cl-(1)This acid is completely dissociated in dilute aqueous solution, which means that the[H3O+] of 0.1 M HCl is 0.1 M.Thus, HCl is a stronger acid than water and completelydonates a proton to water to form H3O+.By experimentation, acetic acid, CH3COOH (abbreviated HOAc), is a weak acid and isnot fully dissociated, as shown by Equation (2):H2O+HOAcH3O++OAc-[2]Its dissociation constant, as shown by Equation (3), is therefore small:+¿H3O¿¿¿O Ac¿¿¿Ka=¿[3]Acetic acid only partially dissociates in aqueous solution, and an appreciable quantity ofundissociated acetic acid remains in solution.For the general weak acid HA, the dissociation reaction and dissociation constantexpression are:HA+H2OH3O++A-[4]+¿H3O¿¿¿A¿¿¿Ka=¿[5]2 |P a g e
Recall that pH is defined as:+¿H3O¿pH=−log¿[6]We, therefore, can rearrange Equation (5) and combine it with Equation (6) in thefollowing way:+¿H3O¿¿A¿¿¿¿¿[7]+¿H3O¿¿KA¿¿¿¿log(¿¿a)−log[HA]¿log¿[8]¿A¿¿¿pH=pKa+log¿[9]During a titration of a weak acid, HA, with a strong base, there will be a point in thetitration where 50 percent of the acid has been titrated to produce A-and 50 percentremains as HA.At this point [HA] = [A-], the ratio [A-]/[HA] = 1, and log[A-]/[HA] = 0.At one-half the equivalence point, Equation (9) becomes:pH=pKa[10]

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Term
Fall
Professor
CARPENTER
Tags
pH, Sodium hydroxide, Mass of Unknown Acid Per

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