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Understanding pH, pOH, and Their Relation to Acidity and Basicity

This article explores the fundamental concepts of pH and pOH, their calculations, and the significance of these values in determining the acidity or basicity of a solution. It defines equilibrium with water ions and gives examples of strong acids and bases, alongside practical applications such as calculating pH for various common substances. Understanding these concepts is valuable for students in chemistry and related fields, as it lays the groundwork for more advanced topics in acid-base chemistry.

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Understanding pH, pOH, and Their Relation to Acidity and Basicity

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  1. pH

  2. Water • Water is in equilibrium with its ions H2O(l)  H+(aq) + OH-(aq) Kw = [H+][OH-] • Kw = 1.0 x 10-14 at 25°C • In neutral solutions [H+]=[OH-]= 1x10-7 • If [H+]>[OH-], solution is acidic • If [H+]<[OH-], solution is basic

  3. Example • Find the [H+] if a) [OH-]= .010M b) [OH-]= 2.0 x 10-9 M

  4. pH • Very small numbers can be conveniently expressed on a log scale • pH = -log [H+] • pH = 7, neutral • pH < 7, acidic • pH > 7, basic

  5. Example • Calculate the pH of lemon juice with an [H+]= 3.8 x 10-4M • Calculate the pH of window cleaner with an [H+]= 5.3 x 10-9M • What is the [H+] of apple juice with a pH of 3.76.

  6. pOH • pOH = -log [OH-] • pOH = 7, neutral • pOH < 7, basic • pOH > 7, acidic • pH + pOH = 14.00

  7. Indicators • Used to estimate pH • Colored substance that exists in either acid or base form • Acid and base form are different colors • By knowing the pH where it changes color, we can determine its relative pH

  8. Strong Acids and Bases • Strong acids: HCl, HBr, HI, HNO3, HClO4, H2SO4 (only the 1st H) • Strong bases: hydroxides of group I and II, except Be

  9. .10M HNO3 .10M H+ • .75M H2SO4  .75M H+ • .50M NaOH  .50M OH- • .75M Ca(OH)2  1.5M OH-

  10. Example • What is the pH of a solution of a) .010M HCl b) .010M Ca(OH)2

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