The definitions of acids and bases have changed over time.

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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

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 –

Bases tend to dissociate into OH- ions in solution.

Example: NaOH (aq) Na+ + OH –

You can think about pH as the “power of hydrogen.”pH measures the concentration of H+ ions in the solution, [H+]

pH = [H+]

pOH is the concentration of OH- ions in solution.

pOH = [OH-]

The pH of water is 7.Water is considered neutral because the number of H+ is equal to the number of OH-.

[H+] = [OH-]

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.

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.

If you know the pH of a solution, what

else do you know?

IV-19: “pHooey!” [H+] and pHCheck-

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