17.4.4 Fractional Precipitation Flashcards Preview

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Flashcards in 17.4.4 Fractional Precipitation Deck (12)
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1
Q

Fractional Precipitation

A
  • A precipitate is formed when the solubility product constant (K sp ) is exceeded.
  • Salts with a common ion and of sufficiently different K sp can be separated.
2
Q

note

A
  • A precipitate is formed when the solubility product constant (K sp ) is exceeded.
  • For example, add silver nitrate (AgNO 3 ) to a 0.10 M Cl – solution:
  • Until [Ag + ][Cl – ] > K sp , nothing happens.
  • Once [Ag + ][Cl – ] … K sp , solid silver chloride precipitates making the solution cloudy.
  • The value of [Ag + ] necessary for the precipitation to occur is 1.5 x 10 –9 M.
  • Salts with a common ion with sufficiently different Ksp can be separated.
  • In the example, the [Ag + ] necessary for the precipitation of silver chloride (1.5 x 10 –9 M) is smaller than that necessary for silver chromate (9.5 x 10 –6 M). Thus, silver chloride will begin to precipitate sooner than the silver chromate. The point at which the silver chromate begins to precipitate can be easily identified by the change in color of the precipitate.
  • The amount of chloride ion remaining in the solution at this point is very small and it has been effectively separated (through precipitation) from the rest of the solution.
  • This method, known as fractional precipitation, can be used as a technique for separation of various compounds from aqueous solution.
  • A table of values for K sp is essential to determine which compounds to use to induce precipitation. The K sp values of the precipitates must be sufficiently different to make this method practicable.
3
Q

Suppose that you have KCl and K2CrO4 in solution, and [Cl − ] = [CrO42− ] = 0.17 M.

When you add enough Ag+ ions to the solution, two reactions eventually occur:

Reaction 1:

(where Ksp = 1.8 × 10−10 )

Reaction 2:

(where Ksp = 9.0 × 10−12 )

Reaction 1 occurs first, because Ksp for Reaction 1 is less than Ksp for Reaction 2. At the exact point when Ag2CrO4(s) just begins to precipitate (Reaction 2), what are the concentrations of [Cl − ] and [Ag+ ] and the percent of Cl − left in the solution?

A

[Cl − ] ≈ 2.5 × 10−5 M;
[Ag+ ] ≈ 7.3 × 10−6 M;
%Cl − = 1.5 10−2 %

4
Q

Suppose you have 0.05 M of B− in a flask. You slowly add A+ ions to the flask. The reaction for A+ (aq) + B− (aq) is as follows:

where Ksp = 1.8 × 10−8.

At what concentration of A+ ( [A+ ] ) will the AB(s) first start to precipitate?

A

3.6 × 10^−7 M

5
Q

Look at the following reaction from the lesson.

(where Ksp = 1.8 × 10−10 )

At which initial concentration of Cl − solution will AgCl(s) begin to precipitate if you have added 1.2 × 10−9M Ag+?

A

initial [Cl − ] = 0.15 M

6
Q

Recall the demonstration in which Ag+ ions were added to a chloride ion solution. At some point, after adding enough Ag+, AgCl(s) precipitated.

(where Ksp = 1.8 × 10−10 )

Which statement about this event is not correct?

A

Sufficient Ag+ was added to the solution depleting the Cl −.

7
Q

Suppose that you have 0.10 M of A+ ions in a flask. You slowly add B− ions to the flask. The reaction for A+ (aq) + B− (aq) is as follows:

where Ksp = 2.3 × 10−10

What is the [B− ] when AB(s) first starts to precipitate?

A

4.8 × 10^−5 M

8
Q

Suppose you have 0.05 M of B− in a flask. You slowly add A+ ion to the flask. The reaction for A+ (aq) + B− (aq) is as follows:

where Ksp = 1.8 × 10−8

At which of the following concentrations for A+ will the AB(s) have already begun to precipitate?

A

3.8 × 10^−6 M

9
Q

Ag+ is added by drops to a solution of chromate, CrO42−.

(where Ksp = 9.0 × 10−12 )

For which initial concentration of chromate anion CrO42− would [Ag+ ] equal to 8.3 × 10−6 cause the solution to begin to precipitate Ag2CrO4(s)?

A

[CrO42− ] = 0.13 M

10
Q

Ag+ is added to a 0.10 M chromate CrO42− solution, eventually yielding silver chromate, Ag2CrO4(s).

(where Ksp = 9.0 × 10−12 )

At which of the following concentrations for Ag+ will the Ag2CrO4(s) have already begun to precipitate?

A

[Ag+ ] = 9.8 × 10−6 M

11
Q

Look at the following reaction.

(where Ksp = 1.8 × 10−10 )

Suppose that you have a Cl − solution of 0.23 M. What is the [Ag+ ] at the point at which AgCl(s) just begins to precipitate?

A

7.8 × 10−10 M

12
Q

Suppose that you have 0.05 M of B− in a flask. You slowly add A+ ions to the flask. The reaction for A+ (aq) + B− (aq) is as follows:

where
Ksp = 1.8 × 10−8

Which statement about this event is not correct?

A

Although [A+ ] and [B− ] will vary as more A+ is added, their product will equal Ksp when AB(s) first begins to precipitate

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