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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

pH and pOH

Chapter 16.2 and 16.3

what is ph and poh
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
calculating ph and poh
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
example
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
more examples
More Examples
  • [H3O+] = 3.4 x 10-5
  • pH = 4.5
  • [H3O+] = 6.89 x 10-3
  • pH = 2.16
using the hydroxide ion
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.
example1
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
calculating h from ph
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
titration
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
titration procedure
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
titration terminology
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.