pH & pOH (College Board AP® Chemistry)

pH & pOH

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is

• The values on the pH scale go from 0 - 14 (extremely acidic substances have values of below 0)

• All acids have pH values below 7, all alkalis have pH values above 7

  • The lower the pH then the more acidic the solution is

  • The higher the pH then the more alkaline the solution is

The pH scale

The pH scale showing acidity, neutrality and alkalinity

pH of acids

• Acidic solutions (strong or weak) always have more H₃O⁺ than OH^- ions

• Since the concentration of H₃O⁺ is always greater than the concentration of OH^- ions, [H₃O⁺] is always greater than 10^-7 M

• Using the pH formula, this means that the pH of acidic solutions is always below 7

• The higher the [H₃O⁺] of the acid, the lower the pH

pH of bases

• Basic solutions (strong or weak) always have more OH^- than H₃O⁺ ions

• Since the concentration of OH^- is always greater than the concentration of H₃O⁺ ions, [H₃O⁺] is always smaller than 10^-7 M

• Using the pH formula, this means that the pH of basic solutions is always above 7

• The higher the [OH^–] of the base, the higher the pH

pH of water

• Water at 298K has equal amounts of OH^- and H₃O⁺ ions with concentrations of 10^-7 M

• To calculate the pH of water, the following formula should be used:

pH = – log₁₀[H₃O⁺ (aq)]

[H₃O⁺ (aq)] = concentration of H⁺/H₃O⁺ ions

pH = -log (10^-7) = 7

• Thus, water has a pH of 7 at 298 K      

pH

• The acidity of an aqueous solution depends on the number of H₃O⁺ ions in the solution

• pH is defined as:

pH = -log₁₀ [H₃O⁺]

• Where [H₃O⁺] is the concentration of H₃O⁺ ions in M

• Similarly, the concentration of H₃O⁺ of a solution can be calculated if the pH is known by rearranging the above equation to:

[H₃O⁺] = 10^-pH

• The pH scale is a logarithmic scale with base 10

  • For example, pH 5 is 10 times more acidic than pH 6

  • This means that each value is 10 times the value below it

• pH values are usually given to 2 decimal places

pH & [H₃O⁺] Table

pOH

• The basicity of an aqueous solution depends on the number of hydroxide ions, OH^-, in the solution

• pOH is defined as:

pOH = -log [OH^–]

• Where [OH^-] is the concentration of hydroxide ions in M

• Similarly, the concentration of OH^- of a solution can be calculated if the pH is known by rearranging the above equation to:

[OH^–] = 10^-pOH

• If you are given the concentration of a basic solution and need to find the pH, this can be done by:

[H₃O⁺] = 

• Alternatively, if you are given the [OH^-] and calculate the pOH, the pH can be found by:

pH = 14^{- pOH}

• As we can see, pH and pOH are interlinked and at all temperatures, pH + pOH = pK_w