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pH and pOH

pH and pOH. Chapter 16.2 and 16.3. What is pH and pOH?. pH is the concentration of hydronium ions in a solution pOH is the concentration of hydroxide ions in a solution pH tells you the strength of an acid or a base (more hydronium = more acidic) Strongest acid = 0.0 Neutral = 7.0

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pH and pOH

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  1. pH and pOH Chapter 16.2 and 16.3

  2. What is pH and pOH? • pH is the concentration of hydronium ions in a solution • pOH is the concentration of hydroxide ions in a solution • pH tells you the strength of an acid or a base (more hydronium = more acidic) • Strongest acid = 0.0 • Neutral = 7.0 • Strongest base = 14.0

  3. Calculating pH and pOH • When you are given either a concentration of hydronium ions or hydroxide ions, you can calculate the pH or the pOH • pH = -log [H3O+] • pOH = -log [OH-] • Each 1 unit change in pH is a 10 fold change in the hydronium ion concentration

  4. Example • What is the pH of a solution with a [H3O+] of 1.0 x 10-12? • pH = -log [1.0 x 10-12] • pH = 12 • What do you notice about the answer and the exponent? • They’re the same! This only works when your hydronium ion concentration is in 1.0 x 10-x form

  5. More Examples • [H3O+] = 3.4 x 10-5 • pH = 4.5 • [H3O+] = 6.89 x 10-3 • pH = 2.16

  6. Using the hydroxide ion • Sometimes you will be given the hydroxide ion concentration and asked to find the pH. • Calculate the pOH • Subtract the pOH from 14 to get your pH • The reverse of this works as well. If you are given a pH and asked to find a pOH, follow the same steps.

  7. Example • What is the pH of a solution that has the hydroxide ion concentration of 0.025 M? • pOH = -log [OH-] • pOH = -log [0.025]= 1.6 • pH = 14 – 1.6= 12.4

  8. Calculating [H+] from pH • If you are given the pH and asked to calculate the [H+], you just need to use you calculator. • Take the inverse log (2nd + log key) • Enter a negative sign (-) • Enter the pH and close parentheses • Example: What is the [H+] for a solution with a pH of 7.41? • [H+] = inverse log (-pH) • [H+] = inverse log (-7.41) = 3.9 x 10-8 • You try!: What is the [H+] for a solution with a pH of 3.50? • [H+] = inverse log (-3.50)= 3.16 x 10-4

  9. Titration • Titration is a method used for determining the concentration of a solution by reacting it with a known volume of another solution of a known concentration • Let’s look at the procedure for titration

  10. Titration Procedure • Measure the amount of the solution of an unknown concentration in a beaker. Add an indicator. • Fill a buret with the titrating solution of a known concentration called the standard solution. • Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker. Continue until the indicator changes

  11. Titration Terminology • Equivalence point – when the moles of hydrogen equal the moles of hydroxide. • This point is not always 7. It may be different when using strong acids vs. weak bases and vise versa. • End point- the point where the indicator changes. This point is different for every indicator.

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