AP Chemistry period 6
Determination of Ka of Weak Acids
The purpose of this lab is to find the strength of weak acids by determining the equilibrium constants for their ionization reactions in water. Is to use their measured pH values to calculate the pKa for the two unknown weak acids thus determining their identities. Hypothesis:
If we neutralize a solution that contains a weak acid by adding a strong base to the solution, then the ions will be isolated and thus finding the concentrations of each species by measuring pH levels and determining [H3O+] possible. Background:
In order to completely ionize the weak acid, a strong base needs to be added to the solution. When the concentration is determined, the calculation of pH levels will be made possible. Since we know that in this experiment, the molar concentrations of an unknown acid and its conjugate base are equal, it can be determined that the pH of the solutions are equal to the pK(a) of the acid. (pH=-log[H30+] and pK(a)=-logK(a)) A crucial piece of information that should be known is that polyprotic acids contain more than one ionizable hydrogen which occurs through many steps that contain their own equilibrium constant. Equilibrium Constant Expression for An Acid:
K(a) = [A-][H3O+]/[HA])
K(a) = [CH3COO-][H3O+]/[CH3COOH] in which acetic acid and acetate ion are equal causing the two to cancel out leaving K(a) = [H3O+] Procedure
1) Label dishes and record unknown letter for an unknown weak acid. 2) Put small quantities of the unknown into each dish.
3) Measure water and combine the water and sample while stirring. 4) Transfer measured acid solution to a flask and add 3 drops of phenolphthalein and sodium hydroxide solution while stirring. 5) Stir and add until pink color persists in the solution.
6) Mix solution by repeatedly pouring contents into a flask and beaker to thoroughly mix. 7) Measure pH level.
8) Dispose of contents...
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