List the major species at points A, B, C, and D on the following
titration curve of the titration of ammonia with HCl.
Figure %: Titration of ammonia with hydrochloric acid
A = NH3, it has yet to be acidified.
B = NH3 and NH4+ in the buffering region.
C = NH4+. At the equivalence point, all the
NH3 has been protonated, and water molecules begin to take up acidic
protons.
D = NH4+ and more acid in solution
(H3O+).
Problem :
Why is it acceptable to use an indicator whose pKa is
not
exactly the pH at the equivalence point?
As we can see in the following titration curve, even if the
pKa of the
indicator is several units away from the pH at the equivalence point, there
is only a negligible change
in volume of titrant added due to the steep slope of the titration curve
near the equivalence point.
Problem :
It takes 26.23 mL of a 1.008 M NaOH solution to neutralize a solution of 5
g of an unknown
monoprotic acid in 150.2 mL of solution. What is the molecular weight of
the unknown?
This is a standard stoichiometry problem for titration. Calculate the
number of moles of base to know the number of moles of the unknown because
it is a monoprotic
acid. Once you know the number of moles of the unknown, divide the mass of the
unknown by the number of moles to obtain the solution: the molecular weight of
the unknown is 189.1 g/mol. Titration stoichiometry
problems do not get much trickier than this.
