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a. The concentration of OH- ions in a sample of seawater is 5.0 x 10-6M. Calculate the concentration of H+ ions and determine the pH and pOH of this solution. Is it neutral, acidic, or basic? b. The concentration of H+ ions in a sample of lemon juice is 2.5 x 10-3M. Calculate the concentration of OH- ions and determine the pH and pOH of this solution. Is it neutral, acidic, or basic?

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To solve this problem we need to keep in mind the definitions of pH and pOH:

  • pH = -log[H⁺]
  • pOH = -log[OH⁻]
  • pH + pOH = 14

a. [OH⁻] = 5.0 x 10⁻⁶ M

With that value we can calculate pOH:

  • pOH = -log (5.0 x 10⁻⁶) = 5.3

With pOH we calculate pH:

  • pH = 14 - pOH = 14 - 5.3 = 8.7

With pH we calculate [H⁺]:

  • 8.7 = -log [H⁺]
  • [tex]10^{-8.7}[/tex] = [H⁺] = 2.0 x 10⁻⁹ M

The pH in the sample is higher than 7.0, so it is basic.

b. [H⁺] = 2.5 x 10⁻³ M

With that value we can calculate pH:

  • pH = -log(2.5 x 10⁻³) = 2.6

With pH we calculate pOH:

  • pOH = 14 - pH = 14 - 2.6 = 11.4

With pOH we calculate [OH⁻]:

  • 11.4 = -log[OH⁻]
  • [tex]10^{-11.4}[/tex] = [OH⁻] = 4.0 x 10⁻¹² M

The pH in the sample is lower than 7.0, so it is acid.

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