Lesson 5.

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Solubility

Lesson 5

Trial Ion Product

We have learned that when two ionic solutions are mixed and if one product has low solubility , then there is a reaction where a precipitate will form .

Pb(NO

3

)

2(aq)

+ 2NaCl

(aq)

→ PbCl

2(s)

+ 2NaNO

3(aq) low solubility

The solubility chart on page 4 predicts this reaction, but only if the solution is 0.10 M or greater . If the molarity is less than 0.10 M , then the reaction may or may not happen.

A trial ion product must be calculated to predict the reaction of all solutions less than 0.10 M .

The capacity of a solution to dissolve a solid is described by the

Ksp.

Pb (NO

3

)

2

Na Cl

Pb 2+ 2Cl -

PbCl

2(s)

Pb 2+ + 2Cl -

The Ksp represents the limit of the solution to dissolve PbCl

2

.

You can add Pb 2+ and Cl until the ion concentrations are equal to the

Ksp .

The solution is saturated and addition PbCl

2 not dissolved.

must sit on the bottom

1.

200.0 mL 0.10 M Pb(NO

3

)

2 is mixed with 300.0 mL of 0.20 M

NaCl, will a precipitate occur?

1.

200.0 mL 0.10 M Pb(NO

3

)

2 is mixed with 300.0 mL of 0.20 M

NaCl, will a precipitate occur?

PbCl

2(s)

Pb 2+ + 2Cl -

Write a dissociation equation for the compound with low solubility.

1.

200.0 mL 0.10 M Pb(NO

3

)

2 is mixed with 300.0 mL of 0.20 M

NaCl, will a precipitate occur?

PbCl

2(s)

Pb 2+ + 2Cl -

0.10 M 0.20 M

Write a dissociation equation for the compound with low solubility.

List the initial molarities of each ion.

1.

200.0 mL 0.10 M Pb(NO

3

)

2 is mixed with 300.0 mL of 0.20 M

NaCl, will a precipitate occur?

PbCl

2(s)

Pb 2+ + 2Cl -

200 0.10 M

500

0.040 M

300

500

0.20 M

0.12 M

Write a dissociation equation for the compound with low solubility.

List the initial molarities of each ion.

Reduce each molarity by the dilution factor: V

1

/V

2

.

1.

200.0 mL 0.10 M Pb(NO

3

)

2 is mixed with 300.0 mL of 0.20 M

NaCl, will a precipitate occur?

PbCl

2(s)

Pb 2+ + 2Cl -

200 0.10 M

500

0.040 M

300

500

0.20 M

0.12 M

TIP

TIP

=

=

=

[Pb 2+ ][Cl ] 2

[ 0.040

][ 0.12] 2

5.8 x 10 -4

Write a dissociation equation for the compound with low solubility.

List the initial molarities of each ion.

Reduce each molarity by the dilution factor: V

1

/V

2

.

Write the Ksp or TIP (trial ion product) and solve.

1.

200.0 mL 0.10 M Pb(NO

3

)

2 is mixed with 300.0 mL of 0.20 M

NaCl, will a precipitate occur?

PbCl

2(s)

Pb 2+ + 2Cl -

200 0.10 M

500

0.040 M

300

500

0.20 M

0.12 M

TIP

TIP

Ksp

=

=

=

=

[Pb 2+ ][Cl ] 2

[ 0.040

][ 0.12] 2

5.8 x 10 -4

1.2 x 10 -5 TIP > Ksp ppt forms

Write a dissociation equation for the compound with low solubility.

List the initial molarities of each ion.

Reduce each molarity by the dilution factor: V

1

/V

2

.

Write the Ksp or TIP (trial ion product) and solve.

Compare to real Ksp

2. Will a precipitate form if 20.0 mL of 0.010M CaCl

2 mixed with 60.0 mL of 0.0080 M Na

2

SO

4

? is

CaSO

4(s)

Ca 2+ + SO

4

2-

20 0.010 M

80

0.0025 M

60 0.0080 M

80

0.0060 M

Ksp

TIP =

TIP =

=

=

[Ca 2+ ][SO

4

2]

[ 0.0025

][ 0.0060]

1.5 x 10 -5

7.1 x 10 -5 TIP < Ksp no ppt forms

3.

Will a precipitate form when equal volumes of 0.020 M CaCl

2 and 0.040 M AgNO

3 are mixed.

The Cl is doubled

AgCl

(s)

Ag + + Cl -

1

2

0.040 M

0.020 M

1

2

0.040 M

0.020 M

TIP

TIP

=

=

=

[Ag + ][Cl ]

[ 0.020

][ 0.020]

4.0 x 10 -4

Ksp =

TIP > Ksp

1.8 x 10 -10 ppt forms

4.

Consider the two saturated solutions AgCl and Ag

2

CrO

4

.

Which has the greater Ag + concentration?

AgCl

Ag + + Cl s s s

Ag s

2

CrO

4

2Ag + + CrO

2s s

4

2-

Ksp =

1.8 x 10 -10 = s 2 s 2 Ksp

1.1 x 10

=

-12 =4s 3

4s 3 s = 1.3 x 10 -5 M

[Ag + ] = 1.3 x 10 -5 M s = 6.5 x 10

[Ag +

-5

Ag

2

CrO

4 has the greater Ag + concentration

M

] = 2s = 1.3 x 10 -4 M

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