Chemistry 104 Study Groups

ACIDS & BASES PROBLEM
ANSWER
 

Exercise

  Consider two acids, HX and HY. HX has Ka = 1.0 x 10-4. HY has Ka = 1.0 x 10-8. Without doing detailed calculations, determine whether a 0.1-M solution of HX or HY would have the higher pH. Without doing detailed calculations, determine whether a 0.1-M solution of X- or Y- would have the higher pH. Calculate all four pH values to confirm the conclusions.

 

Answer

 

The pH of a 0.1-M solution of HY would have a higher pH than a 0.1-M solution of HX. HX is the stronger acid.

The pH of 0.1-M solution of Y- would have a higher pH than a 0.1-M solution of X-. Since HX is a stronger acid than HY, its conjugate base X- is a weaker base than Y-.

 

Explanation

 
  1. This problem can be solved without any calculations. The relative sizes of Ka must be considered.
  2. The relative strength of a pair of conjugate bases is opposite to the relative strength of the acids.

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