Q: What is the minimum $$[Ag^+]$$ required to begin precipitation of $$AgBr$$ from a solution where $$[Br^-] = 0.010\ \mathrm{M}$$ and $$K_{sp}(AgBr) = 5.0 \times 10$$^{-13}$$?

$$5.0 \times 10$$^{-11}$$\ \mathrm{M}$$

Q: If $$Ag^+ is $$added to a solution containing $$[$$CrO_4$$^{2-}] = 0.010\ \mathrm{M}$$, what is the $$[Ag^+]$$ required to begin precipitation of $$Ag_2CrO_4$$? ($$K_{sp}(Ag_2CrO_4) = 1.1 \times 10$$^{-12}$$)

$$1.0 \times 10$$^{-5}$$\ \mathrm{M}$$

Q: At the point when $$Ag_2CrO_4$$ just begins to precipitate ($$[Ag^+] = 1.0 \times 10$$^{-5}$$\ \mathrm{M}$$), what is the $$[Br^-]$$ remaining in solution? ($$K_{sp}(AgBr) = 5.0 \times 10$$^{-13}$$)

$$5.0 \times 10$$^{-8}$$\ \mathrm{M}$$

Question 1

Which of the following best describes the principle behind selective (fractional) precipitation in separating ions from a solution?

Accepted Answer

The ion with the smaller $$K_{sp}$$ value precipitates first because it is less soluble.

Question 2

A solution contains $$[$$CrO_4$$^{2-}] = 0.010\ \mathrm{M}$$ and $$[Br^-] = 0.010\ \mathrm{M}$$. $$AgNO_3 is $$added slowly. Given $$K_{sp}(AgBr) = 5.0 	imes 10$$^{-13}$$ and K$$_{sp}(Ag_2CrO_4) = 1.1 	imes $$10^{-12}$$, which compound will precipitate first?

Accepted Answer

$$AgBr$$

Question 3

What is the minimum $$[Ag^+]$$ required to begin precipitation of $$AgBr$$ from a solution where $$[Br^-] = 0.010\ \mathrm{M}$$ and $$K_{sp}(AgBr) = 5.0 	imes 10$$^{-13}$$?

Accepted Answer

$$5.0 	imes 10$$^{-11}$$\ \mathrm{M}$$

Question 4

If $$Ag^+ is $$added to a solution containing $$[$$CrO_4$$^{2-}] = 0.010\ \mathrm{M}$$, what is the $$[Ag^+]$$ required to begin precipitation of $$Ag_2CrO_4$$? ($$K_{sp}(Ag_2CrO_4) = 1.1 	imes 10$$^{-12}$$)

Accepted Answer

$$1.0 	imes 10$$^{-5}$$\ \mathrm{M}$$

Question 5

During the selective precipitation of $$Br^- as$$ $$AgBr$$, what happens to the $$[Br^-] in $$solution as more $$AgBr$$ forms?

Accepted Answer

It decreases, allowing $$[Ag^+] to $$increase.

Question 6

At the point when $$Ag_2CrO_4$$ just begins to precipitate ($$[Ag^+] = 1.0 	imes 10$$^{-5}$$\ \mathrm{M}$$), what is the $$[Br^-]$$ remaining in solution? ($$K_{sp}(AgBr) = 5.0 	imes 10$$^{-13}$$)

Accepted Answer

$$5.0 	imes 10$$^{-8}$$\ \mathrm{M}$$

Question 7

Why is fractional precipitation effective for separating $$Br^-$$ and $$CrO_4$$^{2-}$$ in the example provided?

Accepted Answer

Because nearly all $$Br^- is $$removed before $$Ag_2CrO_4$$ begins to precipitate.

Question 8

Which of the following statements about $$Q_{sp}$$ and $$K_{sp} is $$correct during selective precipitation?

Accepted Answer

Precipitation begins when $$Q_{sp}$$ equals $$K_{sp}$$ for a given salt.

Question 9

Suppose you want to separate two cations, $$A^+$$ and $$B^+$$, using selective precipitation. $$K_{sp}(ACl) = 1.0 	imes 10$$^{-10}$$ and K$$_{sp}(BCl) = 1.0 	imes $$10^{-7}. $$Which will precipitate first upon addition of $$Cl^-$$?

Accepted Answer

$$ACl$$

Question 10

A chemist wants to separate $$SO_4$$^{2-}$$ and Cl$$^-$$ from a solution using Ba$$^{2+}$$ as the precipitating ion. K$$_{sp}($$BaSO_4) = 1.1$$ 	imes $$10^{-10}$$ and $$K_{sp}($$BaCl_2$$) is $$very high (very soluble). Which anion will precipitate first?

Accepted Answer

$$SO_4$$^{2-}$$

Question 11

If a solution contains $$[Br^-] = 0.010\ \mathrm{M}$$ and $$[$$CrO_4$$^{2-}] = 0.010\ \mathrm{M}$$, what percentage of the original $$Br^-$$ remains in solution when $$Ag_2CrO_4$$ just begins to precipitate? ($$K_{sp}(AgBr) = 5.0 	imes 10$$^{-13}$$, $$[Br^-]$$ at this point is 5.0$$ 	imes $$10^{-8}$$\ \mathrm{M}$$)

Accepted Answer

0.0005%

Question 12

Which of the following best explains why the concentration of the common ion increases slowly during the precipitation of the less soluble salt?

Accepted Answer

The common ion is being removed from solution as the less soluble salt precipitates.

Question 13

In a real-world scenario, why might selective precipitation be used in water treatment plants?

Accepted Answer

To selectively remove toxic metal ions from wastewater by precipitating them as insoluble salts.

Question 14

Which of the following is a necessary condition for successful separation of two ions by selective precipitation?

Accepted Answer

The $$K_{sp}$$ values of their salts must be significantly different.

Question 15

If $$AgNO_3 is $$added to a solution containing $$[Br^-] = 0.010\ \mathrm{M}$$ and $$[$$CrO_4$$^{2-}] = 0.010\ \mathrm{M}$$, what is the sequence of precipitation as $$[Ag^+]$$ increases?

Accepted Answer

$$AgBr$$ precipitates first, then $$Ag_2CrO_4.$$

Question 16

A student claims that selective precipitation cannot achieve complete separation of $$Br^-$$ and $$CrO_4$$^{2-}$$ because some Br$$^-$$ will always remain. Is this correct?

Accepted Answer

No, because the remaining $$Br^- is $$negligible compared to the initial amount.

Question 17

Which calculation would you perform to determine the concentration of an ion remaining in solution at the point when the second salt just begins to precipitate?

Accepted Answer

Use the $$K_{sp}$$ expression for the first salt and solve for the ion's concentration at the $$[common\ ion]$$ required for the second salt.

Selective Precipitation and Fractional Separation of Ions

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