pH and pOH Notes

1
pH and pOH Notes

• Chapter 16. Sections 16.4-16.6

2
Review

• Why can water be both an acid and a base?
• What is the difference between a strong and a
  weak acid?
• http//www.mhhe.com/physsci/chemistry/chang7/esp/f
  older_structure/ac/m2/s1/acm2s1_1.htm

Proton exchange between water molecules.
3
Review Cont

• What is the Bronsted Lowry Definition of an acid
  and a base?
• http//chemmovies.unl.edu/ChemAnime/HYCLWD/HYCLWD.
  html
• Identify the acid, base, conjugate acid, and
  conjugate base of the following, and the pairs
  of the following
• HCl H2O ? H3O Cl-
• Acid Base Conjugate acid Conjugate
  base

4
The pH Scale

• In order to express really, really small numbers,
  chemists often depend on the p scale
• This system is based on logarithms.
• pN - log N
• pN (-1) x log N
• Or we can say you can take the log of a number
  and multiply it by -1

5
The pH Scale cont

• So get out your calculators!
• Lets say I want to express the number of 1.0 x
  10-8 on the p scale
• ON NON GRAPHING CALCULATORS?
• So I enter the number 1.0 x 10-8 on the
  calculator (make sure you use the EE or Exp key
  for x 10)
• The press the log key
• -8.00
• Now you just hit the /- key and you get..
• 8.00

6
The pH Scale cont

• ON GRAPHING CALCULATORS
• Start with hitting the log key
• Then type in 1.0 x 10-8, or 1.0, EE or EXP, -8
• (You may have to enclose this in brackets)
• Hit enter- you should get -8
• The change the sign! 8

7
The pH Scale cont

• Now when we talk about pH, we mean log H
• Or the negative log of the amount of hydrogen
  ions (or protons)
• So to find the pH
• On your GRAPHING calculator
• Press log key
• Enter the amount of H ions
• Change the sign
• On your NON GRAPHING calculator
• Enter the amount of H
• Press the log key
• Press the /- key

8
The pH Scale cont

• So lets try a few!
• Calculate the pH of a solution in which H
  1.0 x 10-9
• Answer 9.00
• Calculate the pH of a solution in which OH-
  1.0 x 10-6
• Oops! we need to find the H!
• Hint H x OH- 1.0 x 10 -14
• Answer 8.00
• Calculate the pH of a solution in which H
  2.10 x 10-5
• Answer 4.68

9
The pH Scale cont

• Because the pH scale is a log scale a change of 1
  number on the scale is actually a power of 10
  change in the amount of H ions
• A pH of 4 has 10 times more H ions than a pH
  of 5, and 100 times more than a pH of 6
• http//www.purchon.com/chemistry/acids.htmanimati
  on

10
The pH Scale cont

• Note the pH decreases as the H increases
• Look at the scale to find several common
  substances

11
The pH Scale cont

• Since p means log of something, you can p of
  other things too
• Like pOH
• pOH -log OH-
• So if OH- 1.00 x 10 -12
• What is the pOH?
• Answer 12.0

12
The pH Scale cont

• Now we can also get a really cool relationship
  between pH and pOH!
• Since H x OH- 1.0 x 10-14
• We could say that -logH x -logOH- -log(1.0
  x 10-14)
• Or
• pH pOH 14.00
• So this makes math WAY easy!!!

13
The pH Scale cont

• So lets say I have something with a pH of 6.00.
  What is its pOH?
• pH pOH 14.00
• 6.00 pOH 14.00
• pOH 14.00 -6.00
• pOH 8.00
• Easy, huh?

14
The pH Scale cont

• We can also do the reverse. If we know the pH, we
  can find the H
• Here is how you do that with your GRAPHING
  calculator
• Hit the inverse log (most calculators are INV or
  2ND then LOG)
• Change the sign of the pH (/-)
• Enter the pH
• Hit enter
• Here is how you do that with your NON GRAPHING
  calculator
• Enter the pH
• Change the sign using the /- key
• Take the inverse log. (most calculators use inv
  or 2nd then log)

15
The pH Scale cont

• Lets try one!
• You are donating blood, and the nurse tells you
  that the pH of your blood is 7.41, what is the
  H for your blood?
• pH 7.41
• Get the negative
• -7.41
• Then get the inverse log
• Answer 3.9 x10-8 M

16
Measuring pH

• The way we usually measure pH is through
  indicators
• These change color is the presence of an acid or
  a base
• Many indicators only work with in a certain range

17
Measuring pH cont...

• We also can use indicator paper to be more
  specific, since it changes color to match a
  specific pH
• We can also measure pH electronically using a pH
  meter which uses voltage to measure the amount of
  H ions

18
Calculating pH of a strong acid

• Remember that a strong acid completely
  dissociates in water.
• For example
• HCl? H Cl-
• So if I have 1.00 M HCl, what M of H do I have?
• Answer 1.00 M
• So can I find the pH?
• pH -log (1.00)
• pH 0
• WOW, strong acid!!!

19
Calculating pH of a strong acid cont

• Try some!
• Calculate the pH .025 M of the strong acid HClO4.
• pH 1.60
• Calculate the pH of 1.2 x 10 -5 M HBr
• pH 4.92

20
Homework

• Read pages 513-524 problems 1-5 on 524, and
  problems 30 and 42 on pages 533-534