The Acid Dissociation Constant, Ka (weak acids)

• • For a weak monobasic aqueous acid, HA. A- is the salt.

  HA (aq) <=> H+ (aq) + A- (aq)

  
• • When dealing with weak acids, you can assume that dissociation of the acid is much greater than dissociation of water.
  • This means you can assume that all the H+ ions come from the acid, so for a monobasic acid [H+]≈[A-].

  
• • The value of Ka is fixed at a given temperature. So if the concentration of one thing in the equilibrium mixture changes then the concentrations of the others must change to keep the value of Ka the same.
  • Changing the temperature of the solution changes the value of Ka. If dissociation is endothermic, warming the solution shifts the equilibrium to the right (it dissociates more), so Ka increases. If dissociation is exothermic, warming the solution shifts the equilibrium to the left, so Ka decreases.
  • A bigger value of Ka means that the acid has dissociated more so it’s stronger.
• Finding the pH of weak acids:

  1. Write out the equilibrium equation first then use it to write an expression for Ka for the weak acid.
  2. Rearrange the equation and substitute in the values for Ka and [HA] to find [H+]^2.
  3. Take the square root of the number to find [H+].
  4. Substitute [H+] into the pH equation to find the pH.
• Finding the concentration of weak acids

  1. Substitute the pH into the inverse pH equation to calculate [H+].
  2. Write an expression for Ka
  3. Rearrange the equation to give the concentration of the acid.
  4. Substitute the values for Ka and [H+] into the equation and solve it.
• Converting Ka into pKa, it works in the same way as calculating pH from the [H+]

  
• Converting pKa into Ka