The definitions of acids and bases
have changed over time.Arrhenius Definition of Acids and Bases:• An acid is any substance that adds
hydrogen ion (H+) to solution.• A base is any substance that adds
hydroxide ion (OH-) to solution.
Because acids and bases add ions to water, they are both electrolytes and will
conduct electricity.
Important: Not all compounds that have OH in them are bases
For example: CH3COOH is acetic acid (vinegar)
IV-18: “Pass the Proton” Acid – Base Theories
Notes
The pH of a solution is determined by:The molarity of the solution ANDThe identity of the solution (acid, base or neutral)
What does pH measure?
IV-19: “pHooey!” [H+] and pHNotes
Acids tend to dissociate into H+ ions in solution.
Example: HCl (aq) H+ + Cl –
IV-19: “pHooey!” [H+] and pHNotes
Bases tend to dissociate into OH- ions in solution.
Example: NaOH (aq) Na+ + OH –
IV-19: “pHooey!” [H+] and pHNotes
You can think about pH as the “power of hydrogen.”pH measures the concentration of H+ ions in the solution, [H+]
pH = [H+]
IV-19: “pHooey!” [H+] and pHNotes
pOH is the concentration of OH- ions in solution.
pOH = [OH-]
IV-19: “pHooey!” [H+] and pHNotes
The pH of water is 7.Water is considered neutral because the number of H+ is equal to the number of OH-.
[H+] = [OH-]
IV-19: “pHooey!” [H+] and pHNotes
pH + pOH = 14Water is neutral.Neutral: [H+] = [OH-]The pH of water is 7, therefore the pOH must be 7 as well.
pH plus pOH adds to 14.
IV-19: “pHooey!” [H+] and pHNotes
pH & pOH scale log scale Each unit of change in pH represents a
tenfold difference in H+ concentrationEX: A solution with a pH of 3 has an
[H+] ten times greater than a solution with a pH of 4
pH = -log [H+]**Same goes for pOH**
pOH = -log [OH-]
The pH number of a solution tells you the concentration of H+ ions in solution.
A pH of 1 corresponds to a H+ concentration of 0.1 moles/L.
[pH of 1= 0.1 M H+]
Solutions that are more acidic have more H+ ions. Solutions that are more basic have less H+ ions.
IV-19: “pHooey!” [H+] and pHNotes
If you know the pH of a solution, what
else do you know?
IV-19: “pHooey!” [H+] and pHCheck-
In
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