pH and pOH

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# pH and pOH - PowerPoint PPT Presentation

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

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
• Strongest base = 14.0
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
• 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
• They’re the same! This only works when your hydronium ion concentration is in 1.0 x 10-x form
More Examples
• [H3O+] = 3.4 x 10-5
• pH = 4.5
• [H3O+] = 6.89 x 10-3
• pH = 2.16
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.
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
• 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 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
• 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
• 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.