CHEM 1A Study Guide - Midterm Guide: Titration, Conjugate Acid, Acid Dissociation Constant

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Acid - base indicator: an indicator changes color with ph because it is form ( h in ) , and another color in its conjugate base form ( in - ) a weak acid that has one color in its acid. State have to be in different color. H ih cag ) t h2o c e , t h , ot lag , Cag , ka = end point : when the concentrations of its acid and base forms are equal. Poly protic acid titrations: many stoichiometric points. Initially naoh reacts completely with the acid to form the di proto conjugate base h . Cag , hz at cag) t hz o ce: at buffer region 1. Ph determined by the proton transfer equilibrium between hsa" ions and hs a " ions produced in the titration. System is at s , while all hs at lost their h.

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Exp 25. pH Measurements-Buffers and Their Properties.

I need help in the calculations for part C finding the pKa in 1,2, and 3. Thanks!

Experiment 25 pH Measurements- Buffers and Their Properties ne of the is its concentration of hydrogen ion. The pecies, on they, the H,O ion) has a great effect on the solubility of many isorganic and ora tions. It is important H ion (or. more more important properties of an aqueous solution conplex metallic cations found in solutions, and on the rates of many chemical reac- of hydrogen ion and understand its effect on solution propertics that we know how to measure the concentration For convenience, the concentration of H" as molarity. The frequendly ex ãion is frequently expressed as the pH of the solution rather than pH of a solution is defined by the following equation: the equation, 4. If the [H to the base 10. If (H'l is 1 x 104 moles per liter, the pH of the solution is, by is 10-? M, the pH is 1.3. tion will always be obeyed: described in terms of pH. In aqueous solutions the following equilibrium rela- In distillecd water [H Soler t lequals IOH 1. so, by Equation 2, IH"] must be I X 10 M. Therefore,the pH of distilled will have a pH 7; if H 1 IOH1. the solution is basic and its pH >7. A solution with a pÅ of 10 will have or 1 Ã 10-4 M. We measure the pH of a solution experimentally in two ways. In the first of these we use a chemical an indicator, which is sensitive to pH. These substances have colors that change over a relatively short two pH units) and can, when properly chosen, be used to determine roughly the pH of a solu- called H range on. Two very common indicators are litmus, usually used on paper, and phenolphthalein, the most common indicator in acid-base titrations. Litmus changes from red to blue as the pH of a solution goes from about 6 to about 8. Phenolphthalein changes from colorless to red as the pH goes from 8 to 10. A given indicator is useful for determining pH only in the region in which it changes color. Indicators are available for measure- ment of pH in all the important ranges of acidity and basicity. By matching the color of a suitable indicator in a solution of known pH with that in an unknown solution, one can determine the pH of the unknown to within about 0.3 pH units. The other method for finding pH is with a device called a pH meter. In this device two electrodes, one of which is sensitive to [H'], are immersed in a solution. The potential between the two electrodes is related to the pH. The pH meter is designed so that the scale will directly furnish the pH of the solution. A pH meter gives much more precise measurement of pH than does a typical indicator and is ordinarily used when an accurate determination of pH is needed Some acids and bases undergo substantial ionization in water, and are called strong because of their essentially complete ionization in reasonably dilute solutions. Other acids and bases, because of incomplete ionization (often far less than even 1% in 0.1 M solution), are called weak Hydrochloric acid, HCl, and sodium hydroxide, NaOH, are typical examples of a strong acid and a strong base. Acetic acid, HCH,0, and ammonia, NH, are classic examples of a weak acid and a weak base. A weak acid will ionize according to the Law of Chemical Equilibrium HB(aq)H'(a) B (aq) 3)

i'm having a lot of difficulties with this exercise, pleasesolve and explain. i appreciate your help and time.

The acid/base indicator HIn undergoes the following reaction indilute aqueous solution: HIn (color 1) < === > H+ + In-(color 2)

The following absorbance data were obtained for a 5.00 x 10-4 Msolution of HIn in 0.1 M NaOH and 0.1 HCl. Measurements were madeat a wavelength of 485 nm and 625 nm with 1.00 cm cell. 0.1 M NaOHA(485 nm)=0.052, A(625 nm)=0.823 0.1 M HCl A(485 nm)=0.454, A(625nm)=0.176

In the NaOH solution, essentially all of the indicator ispresent as In-; in the acidic solution, it is essentially all inthe form of HIn.

a) Calculate mole absorptivities for In- and HIn at 485 and 625nm.

b) Calculate the acid dissociation constant for the indicator ifa pH 5.00 buffer containing a small amount of the indicatorexhibits an absorbance of 0.472 at 485 nm and 0.351 at 625 nm (1.00cm cells)

c) What is the pH of a solution containing a small amount of theindicator that exhibits an absorbance of 0.530 at 485 nm and 0.216at 625 nm (1.00 cm cells).

d) A 25.00 mL aliquot of a solution of purified weak organicacid HX required exactly 24.20 mL of a standard solution of astrong base to reach a phenolphthalein end point. When exactly12.10 mL of the base were added to a second 25.00 mL aliquot of theacid, which contained a small amount of the indicator underconsideration, the absorbance was found to be 0.306 at 485 nm and0.555 at 625 nm (1.00 cm cells). Calculate the pH of the solutionand Ka for the weak acid.

e) What would be the absorbance of a solution at 485 and 625 nm(1.25 cm cells) that was 2.00 x 10-4 M in the indicator and wasbuffered to a pH of 6.000?

CHEM 1A Study Guide - Midterm Guide: Titration, Conjugate Acid, Acid Dissociation Constant

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