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Loose Ends from Bell Work

Learn about pH concepts like indicators, titrations, and buffers, with explanations, examples, and calculations for practical understanding.

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Loose Ends from Bell Work

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  1. Loose Ends from Bell Work • An acidic solution makes the color of pH paper turn ___________________, while a basic solution makes the color of pH paper turn ________________. • The indicator _________________________ is used to study neutralizations of weak acids with strong bases. Red Blue phenolphthalein

  2. Ways to measure pH • Indicators – a compound that can reversibly change color depending on the pH of a solution • pH paper – usually paper that has been soaked in indicators. When dipped in a solution, it will change color to reflect the pH. • pH meters – an electronicinstrument equipped with a probe that can digitally display the pH of a solution.

  3. Titrations

  4. What is a titration? • An experiment used to determine the concentration of an unknown by using a known amount of a solution with a known molarity. • The most common use is determining concentrations of unknownacids.

  5. Titration Terms • Equivalence Point – the point at which the two solutions used in the titration are present in chemicallyequivalent amounts. • Standard Solution – a solution of known concentration • Titrant – A solution of known concentration that is used to titrate a solution of unknown concentration

  6. TitrationCurve

  7. Finding a Suitable Indicator • Every indicator has a transitionrange, the pH range through which an indicator changes color • The indicator selected must change at the appropriate time during the titration. • The instant at which the indicator changes color is the endpoint of the titration. • If the proper indicator was selected, the endpoint and equivalencepoint will be the same.

  8. Calculations • The most important relationship deals with molarity. • Remember M = n/V • Where M = concentration (mol/L), n = moles, and V = volume in L • The resulting formula: • #H+ Macid Vacid = #OH- Mbase Vbase

  9. Examples • If 20.6 mL of 0.010 M aqueous HCl is required to titrate 30.0 mL of an aqueous solution of NaOH to the equivalence point, what is the molarity of the NaOH solution?

  10. Examples • In the titration of 35.0 mL of drain cleaner that contains NaOH, 50.08 mL of 0.409 M H2SO4 must be added to reach the equivalence point. What is the concentration of the base of the cleaner?

  11. Buffers and Ka’s

  12. What are Buffers? • At times, it is essential to maintain a certain pH. • To achieve this, buffers or buffersolutions are used. • A solution made from a weak acid and its conjugate base that neutralizessmall amounts of acids or bases added to it. • It relies on Le Châtelier's principle to “absorb” the excess OH- or H+ ions

  13. Buffers are Everywhere • Buffers act as preservatives, since pH affects taste. • Phosphoric acid is added to food to buffer the pH to help maintain the taste. • Blood has a buffer system. • If the pH of blood varies to far from 7.4, sickness or even death can occur.

  14. Weak Acids and Bases • Weak acids and bases only partially ionize. Therefore, they reach equilibrium. • Since they reach equilibrium, they have equilibriumconstants. • These constants are called acid-ionizationconstants and are abbreviated Ka • As before, products go on top, reactants go on bottom, and solids and liquids are ignored. • Table 7 on page 559 lists some weak acids and their conjugate bases

  15. Helpful Hints • Weak acids will lose one H+ at a time and each loss will have a different Ka constant • This means that the [H3O+] and the other product have the same concentration. • It also means that the coefficients are one for everything in the equation. • Sometimes, the pH will be given, which will need to be changed to [H3O+] first.

  16. You may want to copy this down somewhere • In other words, Ka = [H+][X-]/[HX] • Where [H+] = [X-] • So, Ka = [H+]2/[HX] and the [H+] can come from pH.

  17. Example Problems • Calculate [H3O+] of a 0.150 M acetic acid solution. (Ka on page 559 is 1.75 x 10-5)

  18. Example Problems • Find Ka if a 0.50 M solution of a weak acid has a hydronium ion concentration of 1.3 x 10-4 M.

  19. Example Problems • A solution prepared by dissolving 1.0 mol of benzoic acid in water to form 1.0 L of solution has a pH of 2.1. Calculate the acid-ionization constant.

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