1. pH-pOH-[H+]-[OH-]-Kw-Ka-Kb Acids-Bases 2

2. Autoionization of Water As we have seen, water is amphoteric. In pure water, a few molecules act as bases and a few act as acids. This is referred to as autoionization. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH − (aq)

3. Ion-Product Constant The equilibrium expression for this process is K c = [H 3 O + ] [OH − ] This special equilibrium constant is referred to as the ion-product constant for water, K w. At 25°C, K w = 1.0  10 −14

4. pH pH is defined as the negative base-10 logarithm of the hydronium ion concentration. pH = −log [H 3 O + ]

5. pH In pure water, K w = [H 3 O + ] [OH − ] = 1.0  10 −14 Because in pure water [H 3 O + ] = [OH − ], [H 3 O + ] = (1.0  10 −14 ) 1/2 = 1.0  10 −7

6. pH Therefore, in pure water, pH = −log (1.0  10 −7 ) = 7.00 An acid has a higher [H 3 O + ] than pure water, so its pH is <7 A base has a lower [H 3 O + ] than pure water, so its pH is >7.

7. pH These are the pH values for several common substances.

8. pH Scale Used for small [H + ] solutions pH log scale means exponential 10 scale pH = 0 1 liter of HCl = 1 mole of [H + ]

9. pH Scale For pH 1, what volume would be needed to contain 1 mole of [H + ]? 10 liter trash can

10. pH Scale What about pH of 5, what volume would be needed to supply 1 mole of [H + ] ions? Would need a train boxcar full of slightly acidic solution! (acid rain)

11. pH Scale What about a pH of 10? How much volume would be needed to have 1 mole of [H + ] ions? You would have to gather an hours worth of water from Niagra Falls to get 1 mole [H + ] if it was pH 10!!

12. pH Scale Can you predict how much to get 1 mole of [H + ] from a pH of 14? 25% of Lake Erie! 241 miles long by 57 miles wide… a lot of H 2 O

13. Other “p” Scales The “p” in pH tells us to take the negative log of the quantity (in this case, hydrogen ions). Some similar examples are pOH = −log [OH − ] pK w = −log K w

14. Watch This! Because [H 3 O + ] [OH − ] = K w = 1.0  10 −14, we know that −log [H 3 O + ] + −log [OH − ] = −log K w = 14.00 or, in other words, pH + pOH = pK w = 14.00

15. Important equations pH = −log [H 3 O + ] pOH = −log [OH - ] [H 3 O + ]= 10 -pH [OH - ]= 10 -pOH pH + pOH = 14.00 (pK w @ 25˚C) [H 3 O + ] [OH − ] = K w = 1.0  10 −14

16. How Do We Measure pH? For less accurate measurements, one can use – Litmus paper “Red” paper turns blue above ~pH = 8 “Blue” paper turns red below ~pH = 5 – An indicator

17. How Do We Measure pH? For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

18. a. 1.At pH = 7, [H + ] = [OH – ] 2.At pH = 7, [H + ] = 0 3.At pH = 7, [H + ] = 7 [OH – ] 4.At pH = 7, [H + ] = 7 M

19. b. 1.We can’t predict pH without also knowing [H + ] concentration. 2.The pH does not change as [OH – ] increases because pH = –log[H + ]. 3.The pH increases as [OH – ] increases. 4.The pH decreases as [OH – ] decreases.

20. 1.pH = 17.00 2.pH = 11.00 3.pH = 3.00 4.the pH cannot be determined without [H + ] information.

21. 1.The pH must be greater than 10. 2.The pH must be above about 8. 3.The pH must be equal to 7. 4.The pH must be less than 7.

22. Strong Acids You will recall that the seven strong acids are HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3, and HClO 4. These are, by definition, strong electrolytes and exist totally as ions in aqueous solution. For the monoprotic strong acids, [H 3 O + ] = [acid].

23. Strong Bases Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca 2+, Sr 2+, and Ba 2+ ). Again, these substances dissociate completely in aqueous solution.

24. Sample Problems 1] Because rain washes pollutants out of the air, the lakes in many places are changing. In a New England state, the water in one lake was found to have [H + ] = 3.2 x 10 -5 M. Calculate pH and pOH of the lake’s water and decide if acidic or basic. pH = -log (3.2 x 10 -5 M) pH = -(-4.5) = 4.5 14 – pH = pOH pOH = 9.5 Acidic South Norway & Sweden lakes are without fish because of acid rain!

25. Acid Rain Effect of acid rain on a forest, Jizera Mountains, Czech Republic Particularly badly affected places around the globe include most of Europe (particularly Scandinavia with many lakes with acidic water containing no life and many trees dead) many parts of the United States (states like New York are very badly affected) and South Western Canada. Other affected areas include the South Eastern coast of China and Taiwan. ScandinaviaUnited StatesNew YorkCanadaChinaTaiwan Effect of acid rain on statues

26. Sample Problems 2] What is the pH of a sodium hydroxide solution @ 25˚C that has a concentration of 0.0026 M? NaOH → Na+ + OH- 0.0026` -log (0.0026 M) = 2.6 pH or pOH? pH = 0.0026

27. Sample Problems 3] “Calcerous soil” is soil rich in calcium carbonate (lime). The pH of such soil generally ranges from just over 7 to as high as 8.3. What value of [H + ] corresponds to a pH of 8.3? Is the soil slightly acidic or basic? 10 -pH = [H + ] 10 -8.3 =