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Understanding pH and pOH: Relationship and Calculation in Chemical Equilibrium

This resource provides an overview of pH and pOH as measures of hydronium ion ([H3O+]) and hydroxide ion ([OH-]) concentrations in solutions, respectively. The definitions and calculations for pH (pH = -log[H3O+]) and pOH (pOH = -log[OH-]) are outlined. The relationship between pH and pOH is explained through the water dissociation constant (Kw = [H3O+][OH-] = 1.0 x 10^-14 at 25°C). It discusses how the concentrations of [H+] and [OH-] affect acidity and basicity in solutions, including examples with strong acids and bases.

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Understanding pH and pOH: Relationship and Calculation in Chemical Equilibrium

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  1. Appendix III

  2. pH and pOH October 21, 2011 Equilibrium Unit

  3. pH • A measure of the concentration of [H3O+] in solution pH = - log [H3O+] • Conversely, [H3O+] may be determined if pH of a solution is known. [H3O+] = 10-pH

  4. pOH • Similar to pH, pOH is a measure of the concentration of [OH-] in solution. pOH = - log [OH-] [OH-] = 10-pOH

  5. How are pH and pOH related? H2O(l) <===> H3O+(aq) + OH-(aq) Is the Kw value large or small? How do you know? Kw = [OH-][H3O+] = 1.0 x 10-14 @ 25°C It is mostly water and does not conduct electricity well.

  6. [H3O+] and [OH-] Acids and bases are determined by the relative [H3O+] and [OH-]. • neutral [H3O+] = [OH-] • acid [H3O+] > [OH-] • base [H3O+] < [OH-]

  7. Water Kw = [OH-][H3O+] = 1.0 x 10-14 What are the concentrations of [H+] and [OH-] of pure water? What happens when NaOH is added to water? HCl?

  8. Acids and Bases Interestingly, even in acidic and basic solutions. Kw = 1.0 x 10-14 Therefore, • if [H+] increases, [OH-] decreases • if [H+] decreases, [OH-] increases

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