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How to Calculate pHA From Pkb

The pHA is the pH of the acid. The pH scale can range from zero to 14, but for certain solutions it can be either higher or lower. The pKb is the negative logarithm of the base dissociation constant, or Kb. The pKa is the negative logarithm of the acid dissociation constant, or Ka. It is useful to understand how to calculate the pHA from the pKb because such a process will help you to better comprehend the relationship between these values and the equations that relate them to one another.

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Instructions

- 1
  Determine the pKa of the solution. The relationship between the pKa and the pKb is
  
  pKa + pKb = 14, or pKw, the negative logarithm of the water dissociation constant. For example, if the pKa of methanol is equal to -2.4, then pKa = 14 - pKb = 14 - (-2.4) = 16.4.
- 2
  Write down the information you are given about the concentration of the conjugate base and the acid. For example, the problem may state that you have to find the pH when there are 0.2 M of conjugate base and 0.2 M of acid.
- 3
  Put the values into the Henderson-Hasselbalch equation. The Henderson-Hasselbach equation is pH = pKa + log([conjugate base] / [acid]). Therefore, pH = 16.4 + log (2.0 / 2.0). This gives you an answer of 16.4 for the pH.

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