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Competing ${K}_{sp}$ Values

APCHEM-KE4UJ1

Consider the following dissolution equations with their corresponding ${K}_{sp}$ values.

$$AgCl(s)\rightleftharpoons { Ag }^{ + }(aq)+{ Cl }^{ - }(aq)\qquad { K }_{ sp }=1.6\times { 10 }^{ -10 }$$

$${ Ag }_{ 2 }Cr{ O }_{ 4 }(s)\rightleftharpoons 2{ Ag }^{ + }(aq)+Cr{ O }_{ 4 }^{ 2- }(aq)\qquad { K }_{ sp }=9.0\times { 10 }^{ -12 }$$

As you may know, silver chloride is a white solid. Silver chromate, on the other hand, is brown-red.

A student carries out an experiment to determine the chloride ion content in an artificial seawater sample. She adds a few drops of a sodium chromate solution, ${Na}_{2}Cr{O}_{4}(aq)$ to her seawater sample and then begins slowly adding a standard silver nitrate solution, $AgN{O}_{3}(aq)$, from a buret.

Which of the following BEST describes the changes she will see as she adds the $AgN{O}_{3}(aq)$ to her sample?

A

A white precipitate will begin to form almost immediately. When the slightest bit of orange-red appears, titration is complete.

B

A brownish-red precipitate will begin to form almost immediately. When the last trace of red is gone, titration is complete.

C

A sort of dull-orange precipitate forms at first. When titration is complete, any new precipitate will be white only.

D

A sort of dull-orange precipitate forms at first. When titration is complete, there is no more evidence of a white component in the precipitate.