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Water as an Acid and Base

Water as an Acid and Base. Chapter 15.1C. How can it be?. How can a substance be both an acid and a base? Substances that can behave either as an acid or as a base are called amphoteric substances . Water is the most common amphoteric substance. Ionization of Water.

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Water as an Acid and Base

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  1. Water as an Acid and Base Chapter 15.1C

  2. How can it be? • How can a substance be both an acid and a base? • Substances that can behave either as an acid or as a base are called amphoteric substances. • Water is the most common amphoteric substance.

  3. Ionization of Water • Let’s look at the following equation to understand why water is amphoteric: • H2O + H2O  H3O+ + OH- • In this reaction, one water molecule acts as an acid by donating a proton, the other water molecule acts as a base by accepting the proton. • However, this reaction only happens to a small extent

  4. Concentration of Ions • In any solution you will have a certain concentration of H3O+ and OH- ions. • To demonstrate concentration, we place ions in brackets • Example: [H+] and [OH-] • Note: to simplify we often write H3O+as H+ • The proportion of H3O+ and OH-ions determines whether your solution is acidic or basic

  5. Ion-Product Constant • In a solution, [H+][OH-] = 1.0 x 10-14 • This number is known as Kw, the ion-product constant for water • If your solution has more [H+] than [OH-], the solution is acidic • If your solution has more [OH-] than [H+], the solution is basic. • If [H+] and [OH-] are equal, your solution is neutral • You can use Kw and [H+] or [OH-] to solve for the other

  6. Example 1 • Calculate the [H+] if [OH-]= 1.0 x 10-5 M, and identify whether this solution is acidic, basic, or neutral • [H+][OH-] = 1.0 x 10-14 • [H+][1.0 x 10-5M] = 1.0 x 10-14 • Solve for [H+]: • [H+] = (1.0 x 10-14)/ (1.0 x 10-5)= 1.0 x 10-9 M • Since we have more [OH-] than [H+], the solution is basic.

  7. Example 2 • Calculate [OH-] if [H+]= 10.0 M, and identify whether this solution is acidic, basic, or neutral • [H+][OH-] = 1.0 x 10-14 • [10.0 M][OH-] = 1.0 x 10-14 • Solve for [OH-] • [OH-]= (1.0 x 10-14)/ (10.0 M) = 1.0 x 10-15 M • Since we have more [H+] than [OH-] the solution is acidic.

  8. Book Work! • Complete the following assignment: • P 590 #22, 23

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