pH vs. pOH

The pH and pOH of a compound helps to determine its acidity and basicity. 

The pH Scale

Concept #1: Understanding pH vs. pOH. 

pH stands for the negative logarithmic function of your hydronium concentration, whereas pOH stands for the negative logarithmic function of hydroxide concentration. 

Concept #2: The connection to Hydronium ion and Hydroxide ion concentrations. 

If we know the pH or pOH concentration then we can determine the concentration of hydronium ions or hydroxide ions. 

Concept #3: The pH Scale. 

If the pH is less than 7 then the solution is acidic, if the pH is equal to 7 then the solution is neutral and if the pH is greater than 7 then the solution is basic. 

Concept #4: The pH and pOH connection. 

Under normal conditions when the concentration is less than 1.0 M the pH scale is between 0 to 14. pH and pOH are connected by the following equation: 

Example #1: What is the hydroxide ion and hydronium ion concentration of an aqueous solution that has a pH equal to 6.12?

Practice #1: Which of the following solutions will have the lowest concentration of hydronium ions?

Practice #2: Which of the following statements about aqueous solutions is/are true?

Example #2: A solution is prepared by dissolving 0.235 mol Sr(OH)2 in water to produce a solution with a volume of 750 mL.

 

a) What is the [OH-]?

 

b) What is the [H+]?

Practice #3: What is the Kw of pure water at 20.0°C, if the pH is 7.083?

Auto – Ionization Product

Concept #5: Understanding the Auto-Ionization Reaction.

In a self-ionization reaction two water molecules react with one another, where one acts an acid and the other as a base. 

The ionization product of Kw is used in this self-ionization equation and ignoring solids and liquids gives the  equilibrium equation as: 

Example #3: Determine the concentration of hydronium ions for a neutral solution at 25oC and at 50oC.

pH of Strong Acids and Bases

If we have a STRONG ACID or STRONG BASE then we DO NOT use an ICE chart to find the pH of solution because the yield of H+ and OH  ions are known. 

Concept #6: Calculating the pH of a Strong Acid or Strong Base. 

Example #4: Calculate the pH of a 0.0782 M solution of CaH2.  

Example #5: Calculate the pH of a 0.000550 M HBr solution to the correct number of significant figures.

  1. 3.3
  2. 3.26
  3. 3.260
  4. 3.2596
  5. All are correct

Practice #4: Calculate the pH of 50.00 mL of 4.3 x 10-7 M H2SO4.

pH of Weak Acids and Bases

If we have a WEAK ACID or WEAK BASE then we DO use an ICE chart to find the pH of solution because the yield of H+ and OH  ions are unknown. 

Concept #7: Calculating the pH of a Weak Acid or Weak Base. 

To determine the pH of a weak acid or weak base we must use an ICE Chart. 

Example #6: Pryridine, an organic molecule, is a very common weak base.

 

C5H5N (aq) + H2O (l)  ⇌  C5H5NH+ (aq) + OH- (g)

 

Assume you have a 0.0225 M aqueous solution of pyridine, C5H5N, determine its pH. The Kb value for the compound is 1.5 x 10-9

Example #7: An unknown weak base has an initial concentration of 0.750 M with a pH of 8.03. Calculate its equilibrium base constant. 

Practice #5: Determine the pH of a solution made by dissolving 6.1 g of sodium cyanide, NaCN, in enough water to make a 500.0 mL of solution. (MW of NaCN = 49.01 g/mol). The Ka value of HCN is 4.9 x 10-10.