Chem 2 - Acid-Base Equilibria III: pH, pOH, and pKw

Chem 2 - Acid-Base Equilibria III: pH, pOH, and pKw

1. Acid-Base Equilibria (Pt . 3) pH, pOH , and pK w By Shawn P. Shields, Ph.D. This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution- NonCommercial - ShareAlike 4.0 International License .

2. Recall: Autoionization of Water Water reacts with itself…   base 1 conjugate base 2 conj acid 1 acid 2  

3. Autoionization of Water and K w In pure water at 25 C, K w = 1.0  10 14  

4. Autoionization of Water and K w At 25 C, K w = 1.0  10 14 Therefore, [OH  ] = [H 3 O + ] = 1.0  10 7  

5. pH, pOH , and pK w Recall: pH is related to the concentration of hydronium H 3 O + . pH =  log [ H 3 O + ] The pH of pure water is  

6. pH, pOH , and pK w pOH is related to the concentration of hydroxide OH  . pOH =  log [ OH  ] The pOH of pure water is also  

7. pH, pOH , and pK w Given pH and pOH , how do you think pK w is calculated? p K w =  log K w The p K w of pure water at 25 C is  

8. A Few More Relationships Between pH, pOH , and pK w pH + pOH = pK w pK w =  log [ K w ] = 14 ( at 25  C) pH + pOH = 14 ( at 25 C)

9. Inverse logs for pH, pOH , and pK w pH =  log [ H 3 O + ] 10 pH = [ H 3 O + ] pOH =  log [ OH  ] 10  pOH = [ OH  ] pK w =  log K w = 14 10 pKw = K w Recall: K w (and pK w ) will have different values at temperatures other than 25  C

10. What you Should Be Able to Do Find pOH , given pH and pK w . Find pH , given pOH and pK w Use these relationships to find [ H 3 O + ] and [ OH  ] for various solutions.

11. What you Should Be Able to Do Take the inverse log of the pH to calculate the hydronium ion concentration (H 3 O + ). Use inverse logs to find hydroxide ion concentration (OH  ) or the value for K w at other temperatures.

12. Example calculations will be posted separately. Next up, Calculating the pH of Strong Acids versus Weak Acid Equilibria (Pt 4)