Acids

1
Acids Bases Lesson 5 pH scale
2
H and OH

• There is a way we can connect both H and OH
  together, using what we already know about them.
• We know Kw HOH- 1.0 x 10-14

3
H and OH

• We can create a scale for the concentrations of
  H
• H or H3O
• We can choose to make this scale go from 100 to
  10-14
• remember 100 1

4
HCl 1 M H H2O NaCl OH- NaOH 1 M
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14
This represents H going from 1.0 M to 1.0 x
10-14 M. In other words, going from 1.0 M strong
acid to 1.0 M strong base. OH- 1.0 x
10-14 1.0 x 10-14 1.0 M
H 1.0 x
10-14 Let us simplify those powers of 10
and get them into whole numbers!
5
HCl 1 M H H2O NaCl OH- NaOH 1 M
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14
What if we take the log of -H? We can call
this pH. pH -Log10H p comes from
potenz in German, which means power Hence,
pH represents powers of 10 of H We will
always use logs to the base 10, and so we will
not keep writing 10 as a base anymore.
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14
pH -LogH
Ex pH -Log 10-2 -2
log10 2 x 1 2 If I take -LogH for every
value of H, I get the new scale in blue called
the pH scale, going from 0 - 14 Nice easy
whole numbers This is the pH scale that goes
from 0-14 ( strong acids? strong bases)
7
The pH Scale
pH
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14
pH -LogH For each 1 unit change of
pH, the H changes by a factor of 10 x. This
is the nature of the logarithm. 1. If H
increases by 100 x pH decreases by 2 2. If H
decreases by 1000 x the pH increases by 3 3. If
the pH increases by 2 units the H decreases by
100 x
If the pH is lower than 7, the solution is
acidic! If the pH is greater than 7, the solution
is basic! If the pH is exactly 7, the
solution is neutral!
8
The pH scale has three important end
points.   pH     0 Strong Acid 1.0 M
HCl   7 Neutral H2O or NaCl 14 Strong
Base 1.0 M NaOH
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Match each letter with a formula. All are 1.0
M   NH3   HCl   KOH   HF   H2O

weak base D
strong acid A
strong base E
weak acid B
neutral C
10

• What do we want to take from all of this??

11
pH Equations You must know the following
equations, which are all based on the ionization
of water at 250 C!   H2O ? H OH-   Kw H
OH- 1.00 x 10-14     pH -LogH Or pOH
-LogOH-     pH pOH pKw 14.000
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Some math stuff!We are gonna get you from
here.to here!
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Operation Inverse

-
x

102
Log
2
Log x
10x

• All logs have the base 10. It makes your life
  easy as we deal with powers of 10s all the time
  in our calculations.
• For ex log(103)
  log(0.01) log (10-2)
• log(10 -5)

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One more piece of info. needed

• Log (10M X l0Y ) Log (10M) log (l0Y)
• Very, very important for us
• For ex
• Log ( 3.94 x 10-4 )
• log ( 3.94 ) log ( 10-4 )
• 0.596 - 4
• -3.404

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Why so important?

• Most of our calculations will look like this
• If H3O 3.51 x 10-12 M, what is pH?
• pH -log H or pH -log H3O
• pH -log (3.51 x 10-12 )
• - log (3.51) -log (10-12 )

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Sig Figs and pH
1.00 x 10-4 M
3 sig figs
-Log 1.00 x 10-4
4.000
3 sig figs
The 4 is from the 10-4 which does not count for
sig figs For pH or pOH, only digits after the
decimal are significant
This can also be summarized as when taking the
log of a number, your final answer will have the
same number of decimals places as there are
significant figures in the original number
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Converting from pH to H30 and OH-

• Remember,
• pH -log H, to get H we take the anti-log.
• H anti-log (-pH) On your calculator
  press 2nd then log, then enter (-pH)
• H 10-pH Or
  OH- 10-pOH

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• Examples
• If pH 3.405, what is H and OH- ??
• H 10-pH
  OH- 10-pOH
• pH pOH pKw 14.000

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pH Calculations   H OH- pH pOH 2.5 x
10-4 M
Calculation
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M
Calculation The number of sig figs are 2 (green)
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M
Calculation pH -LogH pH -Log2.5
x 10-4 pH 3.60 The digit before the
decimal does not count as a sig fig. Both digits
after the decimal count for 2 sig figs.
22
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 3.60
Calculation pH -LogH pH -Log2.5
x 10-4 pH 3.60 keep all digits on the
calculator- do not round!
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 3.60
Calculation pH pOH 14.000 3.60 pOH
14.000 pOH 10.40
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 3.60 10.40
Calculation OH- 10-pOH OH- 10-10.40
OH- 4.0 x 10-11 M
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40
Calculation Note that all of the entries in the
last line have two sig figs!
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 9.43
5
Calculation pH pOH 14.000 9.435
pOH 14.000 pOH 4.565
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 9.43
5 4.565
Calculation OH- 10-pOH OH- 10-4.565
OH- 2.72 x 10-5 M
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 2.72
x 10-5 M 9.435 4.565
Calculation HOH- 1.00 x
10-14 H2.72 x 10-5 1.00 x
10-14 H 3.67 x 10-10 M
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565
Calculation Note that all of the entries in the
last line have three sig figs!
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 2.6 x
10-4 M TRY
Calculation pOH -LogOH- pOH
-Log2.6 x 10-4 pOH 3.59
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 2.6 x
10-4 M 3.59
Calculation pH pOH 14.000 pH
3.59 14.000 pOH 10.41
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 2.6 x
10-4 M 10.41 3.59
Calculation H 10-pH H 10-10.41
H 3.8 x 10-11 M
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59
Calculation Note that all of the entries in the
last line have two sig figs!
34
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59
8.40 Try
35
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59 4.0
x 10-9 M 8.40
36
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59 4.0
x 10-9 M 8.40
37
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59 2.5 x
10-6 M 4.0 x 10-9 M 8.40
38
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59 2.5 x
10-6 M 4.0 x 10-9 M 5.60 8.40
39
pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 10.40 3.67 x
10-10 M 2.72 x 10-5 M 9.435 4.565 3.8 x
10-11 M 2.6 x 10-4 M 10.41 3.59 2.5 x
10-6 M 4.0 x 10-9 M 5.60 8.40
Calculation Note that all of the entries in the
last line have two sig figs!
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pH Calculations   H OH- pH pOH 2.5 x
10-4 M 4.0 x 10-11 M 3.60 acid 10.40
3.67 x 10-10 M 2.72 x 10-5
M 9.435 basic 4.565 3.8 x 10-11 M 2.6 x
10-4 M 10.41 basic 3.59 2.5 x 10-6 M 4.0
x 10-9 M 5.60 acid 8.40
If the pH is lower than 7, the solution is
acidic! If the pH is greater than 7, the
solution is basic!
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Homework

• Logs and anti-logs
• p.134 47
• p.135 48
• p. 139 49, 50
• p. 141 55, 56