CHEM 1405

1. CHEM1405 1 CHEM 1405 Class Meeting 24

2. CHEM1405 2 Assignments and Reminders Reading Assignment Chapter 12 by Thursday April 20th Homework Problems due today Thursday Apr 13th Chapter 9 even numbered Problems 2-14 and 20�36 Quiz on acid and bases Today Homework Problems due Thursday Apr 20th Chapter 10 even numbered Problems 2-8 and 18�30, 36, 38, 42-50 Class website http://iws.ccccd.edu/jstankus/

3. CHEM1405 3 Chemistry Help Resources My office hours Tuesdays after class in Lecture room Thursdays 1-2 in Math Lab Free Tutoring through college Students must submit a tutor request form in order to receive detailed information about the available tutoring services.� The form is available on Collin's website and in the following offices: CPC room A108 (ask for Sonia Castillo) PRC room F109 (ask for Shontel Penny or Mary Eldridge) SCC rooms G200 and G141 There�are�group tutoring services available for the following courses: (SUBJECT TO CHANGE!) CHEM 1405, 1411, 1412, 2423, � Also available will be online tutoring in the following courses: (SUBJECT TO CHANGE!) CHEM 1412 & below

4. CHEM1405 4 Exam III Results

5. CHEM1405 5 Objectives What is the relationship between the moles of a solute in a concentrated solution and after it has been diluted? What is an acid-base titration? How is it accomplished? What is the ion product for water? How is it used? What is the pH scale? How can we calculate the concentration of hydrogen ion from a pH value? What is meant by the ionization constant for a weak acid or weak base? How can we calculate the concentration of hydrogen ion from an ionization constant and a given concentration of a weak acid or weak base?

6. CHEM1405 6 Concentrations of Acids and Bases Dilution is a process of producing a more dilute solution from a more concentrated one, by the addition of an appropriate quantity of solvent

7. CHEM1405 7 Dilution Addition of solvent does not change the amount of solute in a solution but does change its concentration

8. CHEM1405 8 Dilution

9. CHEM1405 9 Dilution example What volume of 1.000 M NaOH solution would you use to prepare 500 mL of 0.2500 M NaOH solution?

10. CHEM1405 10 Dilution safety note In diluting acid solutions, always add acid to water. If water is added to acid, the heat of reaction can cause the water to boil, spattering the acid and causing burns.

11. CHEM1405 11 Acid-Base Titrations Titration is a laboratory procedure in which one reactant in solution is added quantitatively to another until the reaction is stoichiometrically complete. It is used to determine the concentration of a solution, such as an acid (reacted with a base).

12. CHEM1405 12 Procedures for a titration A measured volume of a solution of an acid of unknown concentration is transferred to a flask. Then, a solution of a base of known concentration is added carefully from a buret until the reaction of the acid with the base is just complete. The point at which the acid is just neutralized is called the equivalence point of the titration. At that point, the number of moles of OH- added equals the number of moles of H+ that were in the sample of acid

13. CHEM1405 13 IndicatorsOr how to tell when you are at the equivalence point

14. CHEM1405 14 Other indicators Many naturally occurring dyes can be used as acid-base indicators Extracts from things like Beets change from red to purplish in very basic solution. Blackberries, black currants, and black raspberries change from red in acids to dark blue or violet in basic solution. Blueberries change from blue (around pH 2.8-3.2) to red in a strongly acidic solution. Geranium petals contain pelargonin, an anthocyanin which changes from orange-red in acid solution to bluish in basic solution. Red Cabbage

15. CHEM1405 15 More Indicators The color of hydrangea�s depend on the acidity of the soil the are grown in

16. CHEM1405 16 Titration Example A flask contains 20.00 mL of HCl(aq) of unknown concentration. It is just neutralized by addition of 10.25 mL of 0.2010 M NaOH. What is the molarity of the acid? NaOH(aq)�+�HCl(aq)�? NaCl(aq)�+ H2O First calculate moles of NaOH Then use mole ratio from balanced equation to determine moles of HCl

17. CHEM1405 17 Titration Example Continued Calculate Molarity of HCl

18. CHEM1405 18 Water Equilibria Experimentally determined Concentrations [H+] = [OH-] = 1.0 X 10-7 M

19. CHEM1405 19 Ion Product of Water Example The concentration of H+ ions in a sample of lemon juice is 2.5 x 10-3 M. Calculate the concentration of OH- ions.

20. CHEM1405 20 pH scale S�ren P.L. S�renson proposed a simpler method to express acidity than Exponential Notation The pH scale is defined as The negative logarithm of [H+] pH = -log[H+]

21. CHEM1405 21 pH Calculations pH means to represent the concentration of H+ in solution pH = �log [H+]

22. CHEM1405 22 pH Example What is the pH of a solution that has [H+] = 1.0 x 10-4 M? pH = -log[H+] pH = -log (1.0 x 10-4 ) = - (-4.00) pH = 4.00

23. CHEM1405 23 pH Example What is the pH of a solution that has [H+] = 3.7 x 10-2 M? pH = -log[H+] pH = -log (3.7 x 10-2 ) = - (-1.43) pH = 1.43

24. CHEM1405 24 Significant Figures for Logarithms Find the number of significant figures in the number starting number Take the logarithm Report the number of places past the decimal equal to the number of sig figs

25. CHEM1405 25 pH example What is the [H+] in a solution with a pH = 2.79?

26. CHEM1405 26 pH in Common Solutions

27. CHEM1405 27 pH Scale Typical values range from 0 to 14 pH = 7 � neutral pH > 7 � basic pH < 7 � acidic

28. CHEM1405 28 pH Measurement Methods Indicators pH meters

29. CHEM1405 29 Auto-titrators Using a pH meter Acid-Base Titration can be automated

30. CHEM1405 30 pOH Similar to pH we can define pOH pOH = -log[OH-] Relationship between pH and pOH