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Understanding pH and pOH in Aqueous Solutions

Explore the pH and pOH concepts, ion product constant for water, and practice problems to calculate pH and pOH. Learn how to determine if a solution is acidic, basic, or neutral based on its ion concentrations.

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Understanding pH and pOH in Aqueous Solutions

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  1. Ch. 18: Acids & Bases Sec. 18.3: What is pH?

  2. Objectives • Explain the meaning of pH and pOH. • Relate pH and pOH to the ion product constant for water. • Calculate the pH and pOH of aqueous solutions.

  3. Ion Product Constant for Water • Recall.. Pure water contains equal amounts of H+ and OH- because of the self-ionization of water: H2O H+ + OH- • The ion product constant for water (Kw) is [H+][OH-] • At 298 K, the concentrations have been determined to be 1 x 10-7 M. • Therefore, Kw = 1 x 10-14 M2.

  4. Ion Product Constant for Water • [H+][OH-] always equals 1 x 10-14 M2 at 298 K. • This means, if the [H+] increases, then the [OH-] must decrease (and vice versa.)

  5. Practice Problems • The [H+] of an aqueous solution is 1 x 10-5 M. What is the [OH-]? Is the solution acidic, basic or neutral? • If the [OH-] of an aqueous solution is 1 x 10-3 M, what is the [H+]? Is the solution acidic, basic, or neutral?

  6. pH • S.P.L. Sorenson developed the pH scale in 1909. pH stands for “power of hydrogen”. • [H+] is not expressed as a small number in scientific notation but as a logarithm.

  7. pH • pH is defined as the negative logarithm of the [H+] or - log [H+]. • Think of it this way: the pH of a solution is the negative of the exponent on 10 that gives us the [H+]. • Given a solution that has [H+] = 1 x 10-3 M, its pH is -(the exponent on 10) which is -(-3) or 3.

  8. pH This means... • Acids have a pH from 0 to below 7. The lower the number, the stronger the acid. • Neutral (not acidic or basic) is at 7. • Bases have a pH from above 7 to 14. The higher the number, the stronger the base.

  9. Practice Problems Calculate the pH of solutions having the following ion concentrations. • [H+] = 1 x 10-6 M • [H+] = 1 x 10-7 M • [OH-] = 1 x 10-5 M Are these solution acidic, basic, or neutral?

  10. pOH • pOHis the negative logarithm of the [OH-] or -log [OH-]. • Therefore, a solution with [OH-] = 1 x 10-2 M has a pOH of 2. • A simple relationship between pH & pOH makes it simple to calculate one when the other is known: pH + pOH = 14

  11. Practice Problems • If the [OH-] = 1 x 10-3 M, what is the pOH and pH of the solution? Is the solution acidic, basic, or neutral? • A solution has a pH of 8. What are the [H+] & [OH-]? Is the solution acidic, basic, or neutral? • A solution has a pOH of 4. What are the [H+] & [OH-]? Is the solution acidic, basic, or neutral?

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