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# What ${K}_{sp}$ Tells Us

APCHEM-93KEEE

Consider the following solubility equilibria and their respective ${K}_{sp}$ values, all at 298 K.

$$PbCr{ O }_{ 4 }(s)\rightleftharpoons { Pb }^{ 2+ }(aq)+Cr{ O }_{ 4 }^{ 2- }(aq)\qquad { K }_{ sp }=2.0\times { 10 }^{ -16 }$$

$$Pb{ \left( OH \right) }_{ 2 }(s)\rightleftharpoons { Pb }^{ 2+ }(aq)+2{ OH }^{ - }(aq)\qquad { K }_{ sp }=1.2\times { 10 }^{ -15 }$$

$$Zn{ \left( OH \right) }_{ 2 }(s)\rightleftharpoons { Zn }^{ 2+ }(aq)+2{ OH }^{ - }(aq)\qquad { K }_{ sp }=4.5\times { 10 }^{ -17 }$$

$$MnS(s)\rightleftharpoons { Mn }^{ 2+ }(aq)+{ S }^{ 2- }(aq)\qquad { K }_{ sp }=2.3\times { 10 }^{ -13 }$$

Which of the following is the CORRECT statement regarding the solubilities of these compounds?

A

A saturated solution of $Zn{ \left( OH \right) }_{ 2 }$ will have a higher pH than a saturated solution of $Pb{ \left( OH \right) }_{ 2 }$.

B

$PbCr{ O }_{ 4 }$ has the lowest molar solubility in water.

C

The molar solubilities of $PbCr{ O }_{ 4 }$ and $MnS$ are independent of pH.

D

$\left[ { Pb }^{ 2+ } \right]$ is higher in a saturated solution of $PbCr{O}_{4}$ than it is in a saturated solution of $Pb{ \left( OH \right) }_{ 2 }$.