Q: A solution is prepared by mixing 50.0 mL of 0.0200 M Pb(NO$$_3$$)$$_2$$ and 50.0 mL of 0.0300 M NaCl. What is the value of $$Q$$ for PbCl$$_2 in $$the mixed solution?

$$Q = (0.0100)(0.0150)^2$ = 2.25 \times 10$$^{-6}$$

Q: What is the minimum concentration of SO$$_4$$^{2-}$$ needed to begin precipitating Ba$$^{2+}$$ from a solution containing 0.025 M Ba$$^{2+}$$? (K$$_{sp}$$ BaSO_4$ = 1.07$$ \times $$10^{-10})$$

$$[SO_4$^{2-}] = \frac{1.07 \times 10$$^{-10}$$}{0.025} = 4.3 \times 10$$^{-9}$$ M

Q: A solution contains 0.059 M Mg$$^{2+}$$ and you add KOH. What minimum [OH$$^-$$] triggers Mg(OH)$$_2$$ precipitation? ($$K_{sp} Mg(OH$$)$$_2$$ = $$2.06 \times 10$$^{-13}$$)

$$[OH^-] = \sqrt{\frac{2.06 \times 10$$^{-13}$$}{0.059}} = 1.9 \times 10$$^{-6}$$ M

Question 1

Which of the following best describes the relationship between the reaction quotient $$Q$$ and the solubility product constant $$K_{sp} in $$predicting precipitation?

Accepted Answer

Precipitation occurs when $$Q \u003e K_{sp}.$$

Question 2

A solution is prepared by mixing 50.0 mL of 0.0200 M Pb(NO$$_3$$)$$_2$$ and 50.0 mL of 0.0300 M NaCl. What is the value of $$Q$$ for PbCl$$_2 in $$the mixed solution?

Accepted Answer

$$Q = (0.0100)(0.0150)^2$ = 2.25 	imes 10$$^{-6}$$

Question 3

If $$Q \u003c K_{sp}$$ for a given salt in solution, what will happen?

Accepted Answer

The solution is unsaturated and more solid can dissolve.

Question 4

A student mixes 0.010 M AgNO$$_3$$ with 0.015 M KI. Will AgI precipitate? ($$K_{sp}$$ for AgI = $$8.51 	imes 10$$^{-17}$$)

Accepted Answer

Yes, because $$Q \u003e K_{sp}.$$

Question 5

In selective precipitation, which compound will precipitate first when adding Na$$_2SO$_4 to a $$solution containing 0.025 M Ba$$^{2+}$$ and 0.025 M Ca$$^{2+}$$? ($$K_{sp}$$ BaSO$$_4$$ = $$1.07 	imes 10$$^{-10}$$; K$$_{sp}$$ CaSO_4$ = 7.10$$ 	imes $$10^{-5})$$

Accepted Answer

BaSO$$_4$$ will precipitate first because it has a lower $$K_{sp}.$$

Question 6

What is the minimum concentration of SO$$_4$$^{2-}$$ needed to begin precipitating Ba$$^{2+}$$ from a solution containing 0.025 M Ba$$^{2+}$$? (K$$_{sp}$$ BaSO_4$ = 1.07$$ 	imes $$10^{-10})$$

Accepted Answer

$$[SO_4$^{2-}] = \frac{1.07 	imes 10$$^{-10}$$}{0.025} = 4.3 	imes 10$$^{-9}$$ M

Question 7

A supersaturated solution is best described as:

Accepted Answer

A solution in which $$Q \u003e K_{sp}$$ but precipitation is kinetically hindered.

Question 8

What is the correct $$K_{sp}$$ expression for PbBr$$_2$$?

Accepted Answer

$$K_{sp} = [Pb$$^{2+}$$][Br^-]^2$$

Question 9

If you mix equal volumes of two solutions, how do you calculate the new ion concentration?

Accepted Answer

Divide the original concentration by 2.

Question 10

A solution contains 0.059 M Mg$$^{2+}$$ and you add KOH. What minimum [OH$$^-$$] triggers Mg(OH)$$_2$$ precipitation? ($$K_{sp} Mg(OH$$)$$_2$$ = $$2.06 	imes 10$$^{-13}$$)

Accepted Answer

$$[OH^-] = \sqrt{\frac{2.06 	imes 10$$^{-13}$$}{0.059}} = 1.9 	imes 10$$^{-6}$$ M

Question 11

When Ca(OH)$$_2$$ just begins to precipitate from a solution containing Mg$$^{2+}$$ and Ca$$^{2+}$$, what is the remaining concentration of Mg$$^{2+}$$? ($$K_{sp} Mg(OH$$)$$_2$$ = $$2.06 	imes 10$$^{-13}$$; [OH$$^-$$] = 2.16$$ 	imes $$10^{-3} M)$$

Accepted Answer

$$[Mg$$^{2+}$$] = \frac{2.06 	imes 10$$^{-13}$$}{(2.16 	imes 10$$^{-3}$$)^2$$} = 4.4 	imes $$10^{-10}$ M

Question 12

Which of the following is a requirement for effective selective precipitation of two metal ions?

Accepted Answer

The $$K_{sp}$$ values must differ by at least a factor of $$10^3.$$

Question 13

A solution contains 0.0150 M Pb(NO$$_3$$)$$_2$$ and 0.00350 M NaBr. Will PbBr$$_2$$ precipitate? ($$K_{sp}$$ PbBr$$_2$$ = $$4.67 	imes 10$$^{-6}$$)

Accepted Answer

No, because $$Q \u003c K_{sp}.$$

Question 14

After precipitation of AgI from a mixture of AgNO$$_3$$ and KI, which ion remains in excess?

Accepted Answer

I$$^-$$ remains in excess.

Question 15

Which step is NOT part of the method for predicting precipitation when mixing two solutions?

Accepted Answer

Add the $$K_{sp}$$ values of all possible products.

Question 16

What happens when a seed crystal is added to a supersaturated solution?

Accepted Answer

Rapid crystallization occurs and excess solute precipitates.

Question 17

Which of the following is TRUE about the reaction quotient $$Q$$?

Accepted Answer

$$Q is $$calculated using actual concentrations at any moment.

Question 18

In a real-world scenario, why is selective precipitation important in water treatment?

Accepted Answer

It enables the separation and removal of toxic metal ions based on their solubility differences.

Precipitation, Reaction Quotient (Q), and Selective Precipitation in Aqueous Solutions

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