1. pH Scale In this presentation you will:
explore how to measure the strength of acids and bases
Next >

2. Introduction
Acids and bases can be measured with the pH scale which specifies the concentration of hydrogen ions, H+, in a solution.
To show the concentration, the ion is enclosed in square brackets, [H+].
Next >

3. pH values of some common substances
pH Scale
The pH scale is shown for some common substances.
Household Lye
Bleach
Ammonia
Borax
Baking Soda
Sea Water
Blood
Distilled Water
Milk
Corn
Boric Acid
Orange Juice
Vinegar
Lemon Juice
Battery Acid
pH values of some common substances
Milk of Magnesia
Pure (distilled) water has a pH of 7. The strongest acids are around pH 0, and the stronger bases around pH 14.
Next >

4. pH Scale
As the pH scale is logarithmic, change in the acidity or basicity of a solution of one whole pH unit actually represents a change of ten times the concentration of hydrogen ions in the solution.
Household Lye
Bleach
Ammonia
Borax
Baking Soda
Sea Water
Blood
Distilled Water
Milk
Corn
Boric Acid
Orange Juice
Vinegar
Lemon Juice
Battery Acid
pH values of some common substances
H+ concentration, M in moles per liter
Milk of Magnesia
Next >

5. pH Scale
The pH of an acidic or basic solution can be calculated from the concentration of H+ ions in the solution.
Household Lye
Bleach
Ammonia
Borax
Baking Soda
Sea Water
Blood
Distilled Water
Milk
Corn
Boric Acid
Orange Juice
Vinegar
Lemon Juice
Battery Acid
pH values of some common substances
H+ concentration, M in moles per liter
Milk of Magnesia
The pH value is the negative logarithm of the H+ ion concentration:
pH = - log10 [H+]
Next >

6. pH Scale
The concentration of hydrogen ions in most aqueous solutions is between 1×10-14 M and 1 M.
Household Lye
Bleach
Ammonia
Borax
Baking Soda
Sea Water
Blood
Distilled Water
Milk
Corn
Boric Acid
Orange Juice
Vinegar
Lemon Juice
Battery Acid
pH values of some common substances
H+ concentration, M in moles per liter
Milk of Magnesia
Next >

7. pH Scale
Therefore, the range of pH values for these solutions is between 14 and 0:
Household Lye
Bleach
Ammonia
Borax
Baking Soda
Sea Water
Blood
Distilled Water
Milk
Corn
Boric Acid
Orange Juice
Vinegar
Lemon Juice
Battery Acid
pH values of some common substances
H+ concentration, M in moles per liter
Milk of Magnesia
From pH = - log10 (10-14) = 14
to pH = - log10 (1) = 0
Concentration M in moles per liter is sometimes expressed as mol/dm3. These are equivalent terms.
Next >

8. pH Calculations – Strong Acids
A strong acid is one that fully ionizes, or dissociates into its ions, in an aqueous solution.
Ionized
A strong acid is fully dissociated into H+ ions and its associated base ions (known as its conjugate base) in aqueous solution.
Hydrogen Ion
Chlorine Ion
(Conjugate base)
Next >

9. pH Calculations – Strong Acids
The molar concentration of H+ will be equal to the concentration of the acid.
Ionized
This makes pH calculations of strong acids, of known concentration, simple to work out.
Hydrogen Ion
Chlorine Ion
(Conjugate base)
Next >

10. pH Calculations – Strong Acids
HCl(aq) → H+(aq) + Cl-(aq)
In a 0.25 M solution of HCl, the concentration of H+ ions is also 0.25 M.
Hence:
[HCl] = [H+] = [Cl-] = 0.25 mol/L
pH = - log10 [H+]
pH = - log10 [0.25]
pH = 0.6
Next >

11. pH Calculations – Strong Bases
Like strong acids, strong bases are fully dissociated into ions.
H2O(l) ⇌ H+(aq) + OH-(aq)
However, in alkaline solutions, the concentration of H+ ions is extremely low.
The ionic product of water (Kw) is used to calculate the pH of bases.
Kw = [H+] × [OH-] = 1 × 10−14 mol/L
Next >

12. pH Calculations – Strong Bases
Water naturally dissociates into H+ and OH− ions with equal concentrations of 1×10−7 mol/L at 298 K.
H2O(l) ⇌ H+(aq) + OH-(aq)
Kw = [H+] × [OH-] = 1 × 10−14 mol/L
To calculate the concentration of H+ ions in a basic solution, we have to rearrange this equation.
[H+] = Kw / [OH-]
Next >

13. pH Calculations – Strong Bases
Consider a solution of NaOH of 0.25 M concentration at 298 K:
[NaOH] = [Na+] = [OH-] = 0.25
To calculate the ionic product of water:
Kw = [H+] × [OH-] = 1 × 10−14 mol/L
[H+] = 1 × 10−14 / = 4 × mol/L
pH = - log10 [H+]
Therefore, the pH of NaOH is:
pH = - log10 (4 × ) = 13.4
Next >

14. Relationship between pH and pOH
It is also possible to use the concentration of the hydroxide ion (OH-) to measure the strength of an acid or base, as pOH.
Kw = [H+] × [OH-] = 1 × 10−14 mol/L
pH = - log10 [H+]
Next >

15. Relationship between pH and pOH
Example
For a given concentration of hydrogen, say [H+] = 1 × 10−7 mol/L
then, [OH-] = Kw / [H+] = 1 × 10−14 / 1 × 10−7 = 1 × 10−7 mol/L
Since, pH = - log10 [H+], then
pOH = - log10 [OH-]
pOH = - log10 [OH-] = - log10 (1 × 10−7 ) = 7
In this case, pH = 7 and pOH = 7
Next >

16. Relationship between pH and pOH
In all cases, the sum of pOH and pH will equal 14 because of the relationship:
Kw = [H+] × [OH-] = 1 × 10−14 mol/L
Given either pOH or pH, then for any acid or base the other can be calculated.
pOH + pH = 14
Next >

17. Measuring pH
It is often necessary to know the pH of a solution, in order to identify it as acidic or basic.
Additionally, pH changes during a reaction must often be continuously monitored, for example in a titration experiment.
Next >

18. Measuring pH
The most common experimental techniques used for finding pH are:
Indicators, or chemical substances that react to different pH levels by changing color.
Next >

19. Measuring pH
The most common experimental techniques used for finding pH are:
Universal pH indicator paper, which changes color with pH. A chart is used to identify the pH range shown by each color.
pH meter, an instrument which measures the pH directly.
Next >

20. Question 1
The concentration of hydrogen ions in a solution with a pH of 1 is...?
A) twice the concentration of a solution with a pH 2.
B) half the concentration of a solution with a pH 2.
C) ten times the concentration of a solution with a pH 2.
D) a tenth of the concentration of a solution with a pH 2.
Next >

21. Question 1
The concentration of hydrogen ions in a solution with a pH of 1 is...?
A) twice the concentration of a solution with a pH 2.
B) half the concentration of a solution with a pH 2.
C) ten times the concentration of a solution with a pH 2.
D) a tenth of the concentration of a solution with a pH 2.
Next >

22. Question 2
What is the hydroxide ion concentration for a 1 × 10-4 M HCl solution?
A) 1 × 10-4 M
B) 1 × 10-6 M
C) 1 × 10-8 M
D) 1 × M
Next >

23. Question 2
What is the hydroxide ion concentration for a 1 × 10-4 M HCl solution?
A) 1 × 10-4 M
B) 1 × 10-6 M
C) 1 × 10-8 M
D) 1 × M
Next >

24. Summary In this presentation you have seen:
an explanation of the pH scale
pH calculation of an acid and a base
the relationship between pH and pOH
End >