1. Section 6.2 pg. 238 - 244 pH and pOH Calculations

2. Pure Water  Pure water actually ___________________(called “auto- ionization”), so it contains H + (aq) and OH - (aq) ions, but their concentrations are so low that a conductivity test is determined to be negative, or nil.  In a sample of pure water, about two out of every billion molecular collisions are successful in forming hydronium and hydroxide ions  In pure water at SATP, the hydronium ion concentration is very low; about 1 x 10 -7 mol/L This concentration is often negligible and will show no conductivity unless very sensitive equipment is used (pg. 238 – Figure 1)

3. Pure Water  Adding ______ to water adds H + (aq) ions causing the H + (aq) concentration to increase, thus it makes the solution conductive  Adding ______ to water adds OH - (aq) ions causing the OH - (aq) concentration to increase, thus it makes the solution conductive  Aqueous solutions exhibit a wide ______ of hydronium ion concentrations – from more than 10 mol/L for concentrated HCl (aq) to less than 10 -15 mol/L for concentrated NaOH (aq)  This range is called pH; meaning “power of hydrogen”  “the negative of the base ten exponent for the hydronium ion concentration”

4. pH – power of hydrogen  This range is called ____; meaning “power of hydrogen”  “The negative of the base ten exponent for the hydronium ion concentration” 1 x 10 1 mol/L1 x 10 -7 mol/L1 x 10 -15 mol/L pH = -1 pH = 7 pH = 15 Acidic solutionBasic solutionNeutral

5. pH – power of hydrogen [H 3 O + (aq) ] = 10 -pH The pH scale is used to communicate a broad range of hydronium ion concentrations. Most common acids and bases have pH values between 0 and 14

6. pH changes Changes in pH can be deceptive. Adding vinegar to pure water might change the pH from 7 to 4. While this change of 3 pH units may not appear significant, the change in hydronium ion concentration is 10 3 or 1000 times larger

7. Practice  Try pg. 239 #1-3

8. pH Calculations  Do you think solutions always have a pH that is an integer or simply a power of 10?  No, scientists often need pH measurements to one or more decimal places  So our definition of [H 3 O + (aq) ] =10 –pH must be improved so we can convert numbers like 6.7 x 10 -8 mol/L to a pH  Our new definition: -

9. pH Calculations  Sig digs for pH: “The number of digits following the decimal point in the pH value is equal to the number of sig digs in the hydronium ion concentration.” [H 3 O + (aq) ] = 6.7 x 10 -8 (two sig digs) pH = 7.17 (two sig digs)

10. pH Calculations  So from [H 3 O + (aq) ] to pH we use: pH = -log [H 3 O + (aq) ] pH = -log (4.5 x 10 -10 ) pH = 9.35 (two sig digs)  But to go from pH to [H 3 O + (aq) ] we can still use: [H 3 O + (aq) ] =10 –pH [H 3 O + (aq) ] = 10 -9.35 [H 3 O + (aq) ] = 4.5 x 10 -10 mol/L Since pH has no units, the definition of pH includes the requirement that concentration be in mol/L; you will need to add the units to your answer.

11. Using your calculator:  Go to pg. 241 and read the two Learning Tips  Numbers in scientific notation are best entered using the exponent key (EE) – because the calculator treats the entry as one value.  The 10 x key is not recommended because you may obtain the incorrect result in some situations log Try it: Turn [H 3 O + (aq) ] = 4.7 x 10 -11 mol/L into a pH value Calculator: 4.72nd,(-)1 enter1

12. Using your calculator:  A solution has a pH of 5.3. Calculate its hydronium ion concentration.

13. pOH and Hydroxide ion Concentration  Although pH is used more commonly, in some applications it is more practical to describe hydroxide ion concentration.  The definition of pOH follows the same format as pH  Example: Calculate the hydroxide ion concentration of water with a pOH of 6.3. pOH = -log [OH - (aq) ][OH - (aq) ] =10 –pOH

14. Summary  The number of digits _________________________________ in a pH or pOH value is equal to the number of significant digits in the corresponding hydronium or hydroxide concentration.  For both pH and pOH, an _________________ relationship exist between the ion concentration and the pH or pOH. The greater the hydronium ion concentration, the lower the pH is. pOH = -log [OH - (aq) ][OH - (aq) ] =10 –pOH [H 3 O + (aq) ] =10 –pH pH = -log [H 3 O + (aq) ]

15. Practice  Pg. 242 #4-7 (pH)  Pg. 243 #9-11 (pOH)  Pg. 244 #3-6