In the Mohr titration, \(\mathrm{Cl}^{-}(\mathrm{aq})\) is titrated with
\(\mathrm{AgNO}_{3}(\text { aq })\) in solutions that are at about
\(\mathrm{pH}=7\). Thus, it is suitable for determining the chloride ion content
of drinking water. The indicator used in the titration is \(\mathrm{K}_{2}
\mathrm{CrO}_{4}(\text { aq }) .\) A red-brown precipitate of \(\mathrm{Ag}_{2}
\mathrm{CrO}_{4}(\mathrm{s})\) forms after all the \(\mathrm{Cl}^{-}\) has
precipitated. The titration reaction is
\(\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq}) \longrightarrow
\mathrm{AgCl}(\mathrm{s}) .\) At the
equivalence point of the titration, the titration mixture consists of
\(\mathrm{AgCl}(\mathrm{s})\) and a solution having neither \(\mathrm{Ag}^{+}\)
nor \(\mathrm{Cl}^{-}\) in excess. Also, no \(\mathrm{Ag}_{2}
\mathrm{CrO}_{4}(\mathrm{s})\) is present, but it forms immediately after the
equivalence point.
(a) How many milliliters of \(0.01000 \mathrm{M}
\mathrm{AgNO}_{3}(\mathrm{aq})\) are required to titrate \(100.0 \mathrm{mL}\) of
a municipal water sample having \(29.5 \mathrm{mg} \mathrm{Cl}^{-} / \mathrm{L}
?\)
(b) What is \(\left[\mathrm{Ag}^{+}\right]\) at the equivalence point of the
Mohr titration?
(c) What is \(\left[\mathrm{CrO}_{4}^{2-}\right]\) in the titration mixture to
meet the requirement of no precipitation of \(\mathrm{Ag}_{2}
\mathrm{CrO}_{4}(\mathrm{s})\) until immediately after the equivalence point?
(d) Describe the effect on the results of the titration if
\(\left[\mathrm{CrO}_{4}^{2-}\right]\) were (1) greater than that calculated in
part (c) or (2) less than that calculated?
(e) Do you think the Mohr titration would work if the reactants were exchanged
- that is, with \(\mathrm{Cl}^{-}(\text {aq })\) as the titrant and
\(\mathrm{Ag}^{+}(\) aq) in the sample being analyzed? Explain.
