Monday, January 9, 2012

Titration of a weak base with strong acid (Key words: Dissociation constant of ammonia)

Determination of the concentration of ammonia using 0.10M HCl


The pH of the solution at the equivalence point is,

pH = 1/2 pKw -1/2 pKb + 1/2pC

Kb = the dissociation constant of ammonia = 1.85 x 10^-5

At the equivalence point of the neutralization of 0.1M aqueous ammonia with 0.1M HCl,

pH 7 – 2.87 + ½(1.3) = 5.28

For other concentrations, the pH may be calculated from,

If you cant see the image clearly, please click on it.


Pipette out 25.00 mL of the given aqueous ammonia solution into a titration flask, add few drops of methyl orange indicator. Titrate this solution with the 0.100M HCl acid you have prepared.
I. Calculate the concentration of the aqueous ammonia solution.
II. Draw the titration curve
III. Comment on the selection of a suitable indicator for this reaction.
IV. Calculate the pH of the solution at the equivalence point if 100.00 mL of 0.100M methylamine (pKa = 10.64) solution is titrated with 0.100M HCl acid.
V. Calculate the standard free energy change for the above titration at room temperature.


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