Chapter 12

Fundamentals of Analytical Chemistry: 8th ed  ed.

Chapter 12

Chapter 12

12-1

precipitate consists of solid particles with dimensions that are less than 10(a) A colloidal precipitate consists 4

 cm. A crystalline precipitate precipitate consists of solid particles with dimensions that at least 10-4

cm or greater. greater. As a conseuence! crystalline precipitates settle rapidly! rapidly! whereas colloidal  precipitates remain suspended in solution unless caused to agglomerate. (b) "n gravimetric  "n gravimetric precipitation precipitation!! the analyte is con#erted to a sparing solu$le precipitate!

which is then filtered! washed free of impurities! and then con#erted into a product of %nown composition $y suita$le heat treatment. "n gravimetric "n gravimetric volatilization! volatilization! the analyte is separated from other sample constituents $y con#erting it to a gas of %nown composition. (c) Precipitation  Precipitation is  is the process $y which a solid phase forms and is carried out of solution

when the solu$ility product of a chemical species is e&ceeded. Coprecipitation is Coprecipitation is a  process in which normally solu$le compounds are carried out of solution $y precipitate formation. Coagulation! or agglomeration! is the process $y which colloidal particles coalesce to (d) Coagulation! form larger aggregates.  Peptization refers to the process $y which a coagulated colloid re#erts to its original dispersed state. 'eating! stirring and adding adding an electrolyte can coagulate colloidal suspensions. (ashing the coagulated colloid with water often remo#es sufficient electrolyte to permit the re-esta$lishment of repulsi#e forces that fa#or  return to the colloidal state. Occlusion is a type of coprecipitation in which a compound is trapped within a poc%et (e) Occlusion is formed during rapid crystal formation.  Mixed-crystal formation is also a type of coprecipitation in which a contaminant ion replaces an ion in the crystal lattice.

Fundamentals of Analytical Chemistry: 8th ed.

Chapter 12

(f) Nucleation is a process in which a minimum num$er of atoms! ions or molecules

associate to gi#e a sta$le solid.  Particle growth is a process $y which growth continues on e&isting nuclei. )recipitation $y nucleation results in a large num$er of small  particles. )recipitation $y particle growth results in a smaller num$er of large particles.

12-2

(a) Digestion is a process in which a precipitate is heated in the presence of the solution

from which it was formed *the mother liquor +. ,igestion impro#es the purity and filtera$ility of the precipitate. (b) dsorption is the process $y which ions are retained on the surface of a solid. (c) "n reprecipitation! the filtered solid precipitate is redissol#ed and reprecipitated.

ecause the concentration of the impurity in the new solution is lower! the second  precipitate contains less coprecipitated impurity. (d) Precipitation from a homogeneous solution is a techniue $y which a precipitating

agent is generated in a solution of the analyte $y a slow chemical reaction. ocal reagent e&cess does not occur and the resultant solid product is $etter suited for analysis than  precipitate formed $y direct addition of precipitating reagent. (e) /he counter-ion layer  descri$es a layer of solution containing sufficient e&cess

negati#e ions that surrounds a charged particle. /his counter-ion layer $alances the surface charge on the particle. (f) Mother liquor  is the solution from which a precipitate is formed. (g) !upersaturation descri$es an unsta$le state in which a solution contains higher solute

concentration than a saturated solution. upersaturation is relie#ed $y precipitation of e&cess solute.

Fundamentals of Analytical Chemistry: 8th ed. 12-3

Chapter 12

A chelating agent  is an organic compound that contains two or more electron-donor groups located in such a configuration that fi#e- or si&-mem$ered rings are formed when the donor groups comple& a cation.

12-4

"elative supersaturation can $e regulated through control of reagent concentration! temperature and the rate at which reagents are com$ined.

12-5

(a) /here is positi#e charge on the surface of the coagulated colloidal particles. (b) /he positi#e charge arises from adsor$ed Ag ions. -

(c)  3  ions ma%e up the counter-ion layer.

12-6

C' C03' 4



' 2



C' C3' 2





' 20

/he slow hydrolysis of thioacetamide can $e used to generate a source of hydrogen sulfide gas. 'ydrogen sulfide gas is then in#ol#ed in the euili$ria $elow:

' 20  ' 2  '0





' 2

 

 

' 



' 





'0



02





/he 2- generated can then $e used to precipitate 3i2 in the form of 3i.

12-7   Peptization is the process $y which a coagulated colloid returns to its original dispersed

state as a conseuence of a decrease in the electrolyte concentration of the solution in contact with the precipitate. )epti5ation can $e a#oided $y washing the coagulated colloid with an electrolyte solution rather than pure water.

Fundamentals of Analytical Chemistry: 8th ed. 12-8

Chapter 12

Chloroplatinic acid! '2)tCl6! forms the precipitate 7 2)tCl6 when mi&ed with 7  $ut does not form analogous precipitates with i and 3a. /hus! chloroplatinic acid can $e used to separate 7  from a mi&ture containing i and 3a.

12-9

 3ote: M stands for molar or atomic mass in the euations $elow:

(a)

mass 0 2



mass   -a0 4

M 0 

M

-a04

2 M 9g

(b) mass 9g  mass 9g 2 )2   

(c) mass "n  mass "n2  

2

M

9g2 )2

2 M "n M "n

(d) mass 7   mass 7 2 )tCl 6 

2

2 M 7  M

7 2 )tCl6

(e) mass Cu  mass Cu2 *C3+ 2 

2 M Cu M Cu

2 * C3+ 2

(f) mass 9nCl 2



mass 9n  4

(g) mass )$4  mass )$2 

M 

M

9nCl 2

9n  4

M )$  

4

 M )$2

Fundamentals of Analytical Chemistry: 8th ed.

(h) mass ; 2 )2 11  mass )2  : 

M

Chapter 12

;2 )211

M

)2:

(i) mass  3a 2 -2   10' 2   mass -2  

M  3a

2 -4  10 ' 2 

2 M -2

(j) mass  3a 2   mass  3a

0.2=12 g AgCl  

0.2:00 g impure sample

12-11

M

(a)

Al 2



101.=6 g mole

M

 3' 4 Al * 4 + 2



2.1: g mole

Fundamentals of Analytical Chemistry: 8th ed.

0.2001 g Al 2   

1 mole Al 2   101.=6 g Al 2  



.=2:  10  mole  3' 4 Al*4 + 2 

Chapter 12 2 mole  3' 4 Al* 4 + 2 mole Al 2  

2.1: g  3' 4 Al*4 + 2 mole  3' 4 Al*4 + 2

0.=10 g

(b)

0.2001 g Al 2   0.=10 g impure sample

 .=2:  10  mole  3' 4 Al*4 + 2

 100>  102>  3' 4 Al* 4 + 2

 100>  22.0> Al 2  

(c)

no. mole Al  no. mole  3' 4 Al*4 + 2  .=2:  10  mole .=2:  10 2 mole Al 

26.=81 g Al

mole 0.=10 g impure sample

 100>  11.6> Al

12-12

0.:00 g Cu4  :' 2  



41.: g Cu* " + 2 1 mole Cu* " + 2

1 mole Cu4  :' 2  24=.6 g Cu4  :' 2 

 0.828 g Cu* " + 2



1 mole Cu* " + 2 1 mole Cu4  :' 2 

Fundamentals of Analytical Chemistry: 8th ed.

Chapter 12

12-13

0.2000 g Cu4  :' 2  



2 mole 7" 1 mole Cu* "  + 2



1 mole Cu4  :' 2  24=.6 g Cu4  :' 2 

214 g 7" 1 mole 7"



1 mole Cu* "  + 2 1 mole Cu4  :' 2 

 0.42 g 7"

12-14 *3ote: "n the first printing of the te&t! the answer in the $ac% of the $oo% was in error.+

no. mole Ag"  0.:12 g sample  0.201 Al"   .:  10 4 mole Ag" 

24. g Ag" mole Ag"

1 mole Al"  40.0 g Al" 



 mole Ag" 1 mole Al" 

 .:  10 4 mole Ag"

 0.18 g Ag"

12-15 /he precipitate ?2@2; gi#es the greatest mass from a gi#en uantity of uranium.

12-16

Al 2 * C  + 



6 'Cl

0.0:1: g C 2 

 

 

C 2



' 2 



2 AlCl 

1 mole C 2 1 mole Al 2 * C  +  2 mole Al 26.=8 g Al    44.01 g C 2  mole C 2 1 mole Al 2 * C  +  mole Al

 0.0210: g Al 0.0210: g Al  100>  2.60> Al 0.8102 g impure sample

Fundamentals of Analytical Chemistry: 8th ed.

12-17

Cd  2 2

   

Chapter 12

Cd 4

no. mole Cd  no. mole Cd4  0.11 g Cd4 

1 mole Cd4 208.4 g

1 mole Cd



1 mole Cd4

 :.61  10 4 mole /he num$er moles '2 is eual to num$er moles Cd.

mass ' 2  :.61  10 4 mole 

4.08 g ' 2  1 mole ' 2

 0.01=1 g

0.01=1 g ' 2   100>  0.02:> ' 2 :.0 g impure sample

12-18

0.6006 g -aC  

1 mole -aC  1=.4 g



1 mole C 1 mole -aC 



0.2121 g sample

12.011 g C 1 mole C

 100>  1.2> C

12-19

0.1606 g AgCl 

1 mole AgCl 14. g



1 mole C14 ' = Cl : : moles AgCl

:.000 g sample

 1.:8=> C14 ' = Cl :



:4.2 g C14 ' = Cl : 1 mole C14 ' = Cl :

 100>

Fundamentals of Analytical Chemistry: 8th ed.

Chapter 12

12-20 *3ote: "n the first printing of the te&t! the answer in the $ac% of the $oo% was in error.+

mol 'g 2  0.4114 g 'g: * "6 + 2 

1 mole 'g: * "6 + 2 1448.: g 'g: * "6 + 2



: mole 'g 2 1 mole 'g: * "6 + 2

 1.41=8  10 mol 'g 2

  1 mole 'g 2 Cl 2 42.0= g 'g 2 Cl 2   1.41=8  10 mol 'g 2     2 2 mole 'g 1 mole 'g Cl   2 2    100>  41.16> 'g 2 Cl 2 0.8142 g sample

12-21

M

-a * " + 2



48.1 g

M

mole

7"



166.00 g mole

   166.00 g 7"   1 mole -a * " + 2     2 mole 7"        48 . 1 g 1 mole -a * " + mole            2    100>

0.0612 g -a * "  + 2  

1.= g impure sample

 2.12> 7"

Fundamentals of Analytical Chemistry: 8th ed.

Chapter 12

12-22

M  3'





1.006 g mole

M



)t

1=:.08 g mole

 1 mole )t    2 mole  3'     1.006 g  3'           1=: . 08 g 1 mole )t mole              100>  8.4>  3'

0.46= g )t  

0.211: g impure sample



12-23

M

9n2



86.=4 g mole

M

AlCl 



1.4 g mole

 1 mole 9n2     .66  10 mol   86.=4 g  

mol 9n2  *0.644 g  0.:21 g 9n2 +  

    2 mole Cl    1 mole AlCl     1.4 g AlCl     .66  10  mol 9n2            1 mole 9n  mole Cl mole            2      100> 1.1402 g impure sample

 26.24> AlCl 

Fundamentals of Analytical Chemistry: 8th ed.

Chapter 12

12-24 et w  mass of sample in grams

M

-a4

 2.= g B mole

0.200 g -a4  4

M

4

1 mole -a 4



2.= g

8.:  10 mole 4

2



 =6.064 g B mole

2

1 mole 4

1 mole -a4

=6.064 g 4 mole

4

8.:  10 mole 4

 8.:  104 mole 4

 100>  20> 4 2

2

2

 w g sample

w g sample 

2



=6.064 g 4 mole

2

2

 100>  0.412 g sample

20>

/he ma&imum precipitate weight e&pected gi#en this sample weight!

0.412 g sample 

:: g  4

2

100 g sample



1 mole 4 =6.064 g

2



1 mole -a4 1 g 4

2



2.= g -a 4 1 mole

 0.::0 g -a4

12-25 et w  mass of sample in grams.

/he higher percentage of 3i in the alloy sample is selected $ecause this corresponds to ma&imum amount e&pected precipitate.

Fundamentals of Analytical Chemistry: 8th ed. M  3i * 'C

 288.=2 g B mole

4 ' 6 2 3 2 +

0.1: g  3i* 'C 4 ' 6  2 3 2 + 

Chapter 12

M  3i

 :8.6= g B mole

1 mole  3i* 'C 4 ' 6 2 3 2 + 288.=2 g



1 mole  3i 1 mole  3i* 'C 4 ' 6  2 3 2 +

 6.06  10 4 mole  3i 6.06  10 4 mole  3i 

:8.6= g  3i mole

 100>  :>  3i

 w g sample  w g sample 

6.06  10 4 mole  3i 

:8.6= g  3i mole

 100>

:>

 0.102 g sample

12-26

et w  mass of sample in grams (a)

M

AgCl

 14.2 g B mole

0.400 g AgCl 

M

1 mole AgCl