Understanding pH: Lab Preparation & Chemical Alchemy

1. Review of pH and preparation for the pH Lab

2. Carbonic Acid What would the chemical formula be for this molecule? CH2O3

3. When you mix Carbon Dioxide and Water you make Carbonic Acid CO2 + H2O  H2CO3 However, since Carbonic Acid does not hold tightly to the Hydrogen atoms you get a lot of free H roaming around (an acid). H2CO3  HCO3- + H+

4. This is in carbonated drinks.

5. Acetic Acid What would the chemical formula be for this molecule? C2H4O2

6. C2H4O2  (in water becomes) C2H3O2-+ H+ Acid in vinegar

7. pH = the number of H+ per liter of water. So if pH = the amount of H+, then a large pH should be more H+ But it does NOT! How can a bigger number mean less?

8. Let us consider fractions. 1/2 1/3 1/10 Which is the most pie? Notice: as the bottom of the fraction gets bigger the amount gets smaller.

9. The pH value goes on the bottom of a fraction. 1 pH So as the pH value goes up the amount of H+ goes down.

10. However: the pH number is not alone on the bottom of the fraction. The pH value is actually a power of 10 on the bottom of the fraction. 1 pH

11. However: the pH number is not alone on the bottom of the fraction. The pH value is actually a power of 10 on the bottom of the fraction. 1 10pH

12. 1 101 1 10 pH 1 = = 1 103 1 1,000 = pH 3 = 1 1010 1 10,000,000,000 pH 10 = =

13. But this does not make any sense! pH 1 = 1/10 H+ per liter of water. How do you have 1 tenth of a H+ per liter of water?

14. There is one thing forgot to tell you! It is not 1/10 of a H+ It is 1/10 of a “gram” of H+ “gram” = 600,000,000,000,000,000,000,000 = 6 x 1023

15. A 0 pH solution would have 1 10pH Gram of H+ per liter. 1 100 Gram of H+ per liter.

16. A pH 0 solution 1 100 Gram of H+ per liter. 1 1 Gram of H+ per liter.

17. A pH 0 solution 1 1 Gram of H+ per liter. 1 Gram of H+ per liter.

18. A pH 0 solution gram = 6 x 1023 1 x 1 Gram of H+ per liter. 6 x 1023 H+ per liter.

19. A pH 0 solution So, a pH 0 solution has 6 x 1023 H+ in each liter. 1 x 6 x 1023 H+ per liter.

20. A pH 0 solution would have 1 Gram of H+ per liter. 100 Or 6 x 1023 H+ per liter.

21. pH of 1 = 1/101 grams of H+ per liter x 6 x 1023 1 101 Any math people out there?

22. 1 x 6 x 1023 101 6 1023 x x 101 1 1 6 1022 x x 1 1 1

23. 1 x 6 x 1023 101 6 1023 x x 101 1 1 6 1022 x x 1 1 1

24. 6 x 1022 H+ per liter 6 1022 x x 60,000,000,000,000,000,000,000 1 1 1

25. Even a liter of pH 14 (a strong base) has a lot of H+ in it.

26. pH 14 x 6 x 1023 1 1014 pH 6 x 1023 6 x 109 1014 1 6,000,000,000 H+ per liter of water.

27. A pH 7 (neutral) would have. 6 x 1016 H+ per liter So how can something with so many H+ ions NOT be highly reactive, how can it be neutral? It is balanced out by an equal amount of OH- (hydroxide ions).

28. Measuring pH: But not today

29. We are going to use Indicators: chemicals that change color based on the pH of the solution they are in.

30. Actually we are going to use papers that have be soaked in indicators = pH paper

31. measuring½ ml using the pipets.

32. 1. Using a pipit, put ½ ml of a solution in wells #1,2,and 3 of your chem-plate. (Be sure to put the same chemical in all three wells)

33. You should never have more than one chemical at your table. All the same chemical.

34. If you have more than 1 ph paper at your table then I will subtract 10 from your score.

36. 2. Dip a small piece (about ½ cm) of pH paper into well # 1 (using tweezers) and then remove. Using the pH color card, determine the pH value before the strip has time to dry, and record the pH value in your chart above. pH = 10

37. Do NOT use this big color chart with for determining pH.

38. pH = 10

39. 10

41. 10 Base

43. 10 Base Medium

44. Phenolphthalein Phen

45. D4

46. 10 Base Medium D4

47. Bromthymol Blue B B

48. A5

49. 10 Base Medium D4 A4

50. Universal Indicator U I