Experiment 12: Intro to Qualitative Analysis

Question 1

Detecting NH4+

Answer 1

Moist red litmus paper turns blue
NH4+(aq) + NaOH(aq) -> NH3(g) + H2O(l) + Na+(aq)

Question 2

Detecting Ag+

Answer 2

Adding HCl(aq); White solid
Ag+(aq) + HCl(aq) -> H+(aq) + AgCl(s)

Question 3

Confirming Ag+

Answer 3

1.) Adding NH3+(aq) to rule out Hg+ and Pb+
AgCl(s + NH3+(aq) -> [Ag(NH3)2]+(aq) + Cl-(aq)
2.) Adding HNO3(aq) until solution is acidic will re-precipitate Ag+
[Ag(NH3)2]+(aq) + 2HNO3(aq) + Cl-(aq) -> AgCl(s) + 2 NH4NO3(aq)

Question 4

Detecting F3+, Al3+, Cr3+ (Overall)

Answer 4

Adding an ammonia/ammonium buffer will make a precipitate; the buffer solution increases the concentration of OH- ions forming solid hydroxides
- Fe3+(aq)+ 3NH3(aq) + 3H2O(l) -> Fe(OH)3(s) +3NH4+(aq)
- Cr3+(aq) + 3NH3(aq) + 3H2O(l) -> Cr(OH)3(s) + 3NH4(aq)
- Al3+(aq) +3NH3(aq) + 3H2O(l) -> Al(OH)3(s) + 3NH4+(aq)

Question 5

Separating Fe3+

Answer 5

Fe3+ is separated from Cr3+ and Al3+ by adding a NaOH (strong base) and H2O2 (hydrogen peroxide). Doing so will dissolve Al3+ and Cr3+
- 2Cr(OH)3(s) + 3H2O2(aq) -> 2CrO42-(aq) (Chromate) + 8H2O(l)
- Al(OH)3(aq) + OH-(aq) -> [Al(OH)4]- (aq)
- Fe(OH)3(s) + OH-(aq) -> NR

Question 6

Confirming Fe3+

Answer 6

Dissolving Fe3+ ppt. with HCl(aq). Then adding potassium hexacyanoferrate (II), K4[Fe(CN)6]3(aq), Fe3+is confirmed upon the formation of a deep blue(prussian blue) suspension
- Fe(OH)3(s) + 3H+(aq) -> Fe3+(aq) + 3H2O(l)
- 4Fe3+(aq) + 3Fe(CN)6-(aq) -> Fe4[Fe(Cn)6]3(s)

Question 7

Separating Al3+ from Chromium

Answer 7

With the supernatant with the dissolved CrO42-(aq) and [Al(OH)4]-(aq), heat solution until dryness to remove excess H2O2. Then rehydrate dried residue with HCl(aq) forming dichromate ion C2O72- and Aluminum ion Al3+.
- 2CrO42-(aq) + 2H+(aq) -> CrO72-(aq) + H2O(l)
- [Al(OH)4]-(aq) + 4H+(aq) -> Al3+(aq) + 4H2O(l)

Question 8

Confirming Al3+

Answer 8

Adding ammonia, NH3+ to the solution forms a clear, glass-like ppt. Then dissolve the precipitate with HCl(aq). Add aluminon reagent and then add ammonia to re-precipitate an Al(OH)3(s)
- Al3+(aq) + 3NH3(aq) + 3H2O(l) -> Al(OH)3(s) + 3NH4+(aq)
- Al(OH)3(s) + 3H+(aq) ->Al3+(aq) + 3 H2O(l)
- Al3+(aq) + 3NH3(aq) + 3H2O(l) + Aluminon -> Al(OH)3(s) (red lake) + 3NH4+(aq)

Question 9

Confirming Cr3+

Answer 9

Adding BaCl2(aq) to supernatant (should be yellow) and will form BaCrO4, yellow barium chromate ppt.
-BaCl(aq) + CrO42-(aq) -> BaCrO4(s) Cl-(aq)

Question 10

Confirming Ca2+

Answer 10

1.) Adding Ammonium oxalate (NH4)C2O4(aq) forms white ppt of CaC2O4(s).
-Ca2+(aq) + C2O42-(aq) -> CaC2O4(s)
2.) Dissolving ppt with HCl(aq) and perform flame test - should produce a bright red flame
-Ca2C2O4(s) + 2H+(aq) -> Ca2+(aq) + H2C2O7(aq)

Question 11

Separation of Zn2+ from Mg2+ and Ni2+

Answer 11

1.)Evaporating solution and then adding 16M HNO3 to remove excess ammonium ions
-NH4Cl(s) -> NH3(aq) + HCl(aq)
-NH4NO3(s) -> N2O (g) +2 H2O(l)
2.)Adding NaOH results in Ni2+ and Mg2+ ppt. Zinc becomes a complex ion because it’s amphoteric.
- Ni2+(aq) + 2OH-(aq) -> Ni(OH)2(s)
- Mg2+(aq) + 2OH-(aq) -> Mg(OH)2(s)
-Zn2+ (aq) + 4OH-(aq) -> [Zn(OH)4]2- (aq)

Question 12

Detection of Zn2+

Answer 12

Adding K4[Fe(CN)6] (aq), potassium hexacyanoferrate (II) forms a deep blue precipitate Zn3K2[Fe(CN)6]2(s)
- 3[Zn(OH)4]2-(aq) + 12H+(aq) + K4Fe(CN)6 -> Zn3K2[Fe(CN)6]2(s) +12 H2O(l)

Question 13

Separating Ni2+ and Mg2+

Answer 13

Adding ammonia to the supernatant will react with Ni(OH)2(s) to form complex ion [Ni(NH3)6]2+ (aq). Mg(OH)2(s) remains solid.
- Ni(OH)2(s) + NH3 (aq) -> [Ni(NH3)6]2+(aq) + 2OH-(aq)

Question 14

Detecting Ni2+

Answer 14

Adding dimethylglyoxime to the supernatant, which forms a strawberry red ppt.
- [Ni(NH3)6]2+(aq) + 2 HC4H7N2O2 (aq) -> Ni(C4H7N2O2)2 (s) + 4NH3(aq) + 2NH4+(aq)

Question 15

Detecting Mg2+

Answer 15

1.) Dissolve Mg(OH)2(s) with HCl(aq)
Mg(OH)2(s) + 2 H+(aq) -> Mg2+(aq) + 2H2O(l)
2.) Adding Mg Reagent and NaOH until basic to form “blue lake”
Mg2+ + Mg Reagent + 2OH-(aq) -> Mg(OH)2(s)

Question 16

What is a buffer?

Answer 16

A buffer is a solution of an acid and its conjugate base that is in equilibrium.

Question 17

What does amphoteric mean?

Answer 17

Can react either with an acid or a base

Question 18

What is a lake?

Answer 18

A lake refers to a precipitate that is formed in the presence of an indicating dye like aluminon. The dye absorbs to the precipitate turning it a color. This indicates the precipitate contains the ion indicated by the dye.