CHEM 21131 Physical Chemistry Laboratory
2014
Pre Lab–Experiment C6
Colourimetric determination of the equilibrium constant, Ka, of an organic acid
1. Bromothymol blue, is an organic acid:
bromothymol blue (aq) ⇌ conjugate-base anion (aq) + H+ (aq).
In acidic solution (low pH), will bromothymol blue exist mainly as its parent molecule
(acid) form or mainly as its conjugate-base anion form? Briefly justify your answer.
2. In basic solution (high pH), will bromothymol blue exist mainly as itsparent molecule
(acid) form or mainly as its conjugate-base anion form?
Briefly justify your answer.
3. A solution has pH=4.49. What is the concentration of [H+] = [H3O+]?
4. Consider the following equilibrium between an acid and its conjugate-base:
acid (aq) + H2O(l) ⇌ conjugate-base anion (aq) + H3O+ (aq).
The acid/conjugate-base mixture has 0.21 mole fraction of the acid form.
What is the ratio of concentrations [conjugate-base anion]/[acid]?
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