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

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 electronic instrument equipped with a probe that can digitally display the pH of a solution.

Titrations

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 unknown acids.

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

Titration Curve

Finding a Suitable Indicator Every indicator has a transition range, 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 end point of the titration. If the proper indicator was selected, the end point and equivalence point will be the same.

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

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?

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?

Buffers and Ka’s

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

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.

Weak Acids and Bases Weak acids and bases only partially ionize. Therefore, they reach equilibrium. Since they reach equilibrium, they have equilibrium constants. These constants are called acid-ionization constants 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

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.

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.

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

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.

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.