HÓA PHÂN TÍCH giai BT chuong 6 chuẩn độ oxi hóa

6.1 - electron

Fe2+ + MnO4- + H+ Fe3+ + Mn2+ + H2O

C2O42- + MnO4- + H+ Mn2+ + H2O + CO2

H2O2 +MnO4- + H+ Mn2+ + H2O + 5O2

NO2- +MnO4- + H+ Mn2+ + H2O +NO3

-Cr2O72- + I- + H+ Cr3+ + I3- + H2O

Fe3+ + I- I3- + Fe2+

I3- + S2O32- 3 I- + S4O62

a Fe2+ + MnO4- + H+ Fe3+ + Mn2+ + H2O

Fe2+ - 1e Fe3+ x 5

MnO4- + 8H+ + 5e Mn2+ + 4H2O x 1

5Fe2+ + MnO4- + 8H+ 5Fe3+ + Mn2+ + 4H2O

b C2O42- + MnO4- + H+ Mn2+ + H2O + CO2

MnO4- + 8H+ + 5e Mn2+ + 4H2O x 2

C2O42- -2e 2 CO2 x 5

5C2O42- + 2MnO4- + 16H+ 2Mn2+ + 8H2O + 10CO2

c H2O2 +MnO4- + H+ Mn2+ + H2O + 5O2

MnO4- + 8H+ + 5e Mn2+ + 4H2 +) x 2

H2O2 - 2e O2 +) x 5

+ +

5H2O2 +2MnO4- + 6H+ 2Mn2+ + 8H2O + 5O2

d NO2- +MnO4- + H+ Mn2+ + H2O +NO3

-MnO4- + 5e Mn2+ x 2

NO2- -2e NO3- x 5

2MnO4- + 5NO2- 2Mn2+ + 5NO3

+

2MnO4- + 5NO2- + 6H+ 2Mn2+ + 5NO3+ 3H2O

e Cr2O72- + I- + H+ Cr3+ + I3- + H2O

Cr2O72- + 6e + 14H+ Cr3+ + 7H2O x 1

2I- -2e 2

I2 + I- 3-

Cr2O72- + 9I- + 14H+ 2Cr3+ + 3I3- + 7H2O

f Fe3+ + I- I3- + Fe2+

3I- - 3- x 1

Fe3+ 2+ x 2

2Fe3+ + 3I- I3- + 2 Fe2+

g I3- + S2O32- 3 I- + S4O6

2-3I- - 3- x 1

2S2O32- + 2e S4O62 x 1

I3- + 2 S2O32- 3 I- + S4O62

6.2

a.CrO2- , H2O/Cr, OH-

b.SO42-,H+/H2SO3, H2O

c.AsO43-,H+/AsO33-,H2O

a CrO2- + 3e + 2H2O

-3I- - 3- x 3

b, SO42- +2e + H2SO3 (thi u 2H+ và th a 1oxy)

SO42- +2e + 4H+ H2SO3 + H2O

c AsO43-,H+/AsO33-,H2O

AsO43- + 2e + 2H+ AsO33- + H2O

I2/I-)=0,54V

AsO34 + 2I- + 2H+ AsO33 + I2 + H2O

AsO34 + 2e + 2H+ AsO33 + H2O

2I- - 2e I2

AsO34 / AsO3

3

E = Eo + lg [H+]2 + lg

] ]

[AsO34 ] = [AsO33 ] = 1(M) và [H+]= 100 (M)

dktc E0 (H3AsO4/ H3AsO3)= 0,559 V > E0( I2/I-)=0,54V

Khi pH=8 E = Eo + lg [H+]2 + lg

]

]

o

khi:

[AsO34 ] = [AsO33 ] = 1(M) và [H+]= 10-8 (M)

Eo'= Eo pH = 0,57 0,059 × 8 = 0,098V

y pH =8 , [AsO34 ] = [AsO33 ] = 1(M) o=0.098v < E0( I2/I-)=0,54V Nên I2

AsO33 Ph n ng trên s x y ra theo chi c

Ag+/Ag Cl

-+ + 1e Ag

(1)

- Ag+ + Cl- AgCl

1e Ag + Cl-

(2)

Trong du

{AgCl / Ag, Cl- } và là th chu u ki n c a c p

Ag+/Ag khi có m t Cl-

=>

-4

MnO4- + 5e- + 8H+ Mn2+ + 4H2O MnO4=MMnO4/5

MnO4- + 3e- + 4H+ MnO2 + 2H2O MnO4=MMnO4/3 MnO4- + 3e- + 2H2O MnO2 -

-MnO4- + 1e- 42- MnO4=MMnO4/1

KMnO4

KMnO4

2

4

4 2

4 :

KMnO4 + H2O = MnO2 + 4KOH + 3O2 (1)

KMnO4

2

4

-

- (VN) = (VN)s

325ml H2SO4

6.12 2C2O4 0,05N thêm vào 5ml H2SO4

4

NKmnO4 = (NV)H2C2O4 /VKmnO4 =0.0515N

2+ 5Fe2+ + MnO4- + 8H+ 5Fe3+ + Mn2+ + 4H2O

TKNMnO4/Fe = NKmnO4 Fe/1000 = 2.8896 /1000 = 2.9 mg/l

2O2

5Fe2+ + MnO4- + 8H+ 5Fe3+ + Mn2+ + 4H2O 5C2O42- + 2MnO4- + 16H+ 2Mn2+ + 8H2O + 10CO2 5H2O2 + 2MnO4- + 6H+ 2Mn2+ + 8H2O + 5O2

6.14

4 0,1000N?

Fe + H2SO4 = FeSO4 + H2 3+)

4 5Fe2+ + MnO4- + 8H+ 5Fe3+ + Mn2+ + 4H2O

4

-Fe x (NV)KmnO4 x 100]: mmau = [56 x 10-3 x (24.85 x 0.1) x 100 ]/0.14 =99.4%

30 ml KMnO4 0,1N

Fe2O3 Fe2+ KMnO4

KmnO4 x 10-3 x Mfe

2O3 KmnO4 x 10-3 x Mfe x (MFe2O3/2MFe)

KmnO4 x 10-3 x (MFe2O3/2) x(100/a) (a : %Fe2O3 có trong

VKmnO4=20ml mquang = 0.2286g

VKmnO4=30ml mquang = 0.3429g

và H2SO4

4

2C2O4

4

C2O42- + MnO2+ 4H+ Mn2+ + 2H2O + 2CO2

5C2O42- + 2MnO4- + 16H+ 2Mn2+ + 8H2O + 10CO2

- 2 MnO2 x [(NV)H2C2O4 (NV)MnO4]

- (MMn x10-3/2)x [(NV)H2C2O4 (NV)MnO4]

% Mn = (MMn x10-3/2)x [(NV)H2C2O4 (NV)MnO4]x 100/mmau

= 6,875%

C2O42- + Ca2+ = CaC2O4

Tách k t t a chu n H2C2O4

5C2O42- + 2MnO4- + 16H+ 2Mn2+ + 8H2O + 10CO2

mCa+ = (MCa x10-3/2)x [(NV)H2C2O4 (NV)MnO4] x (250/25) = 0.74g

6.18

Na2S2O3

25ml?

Cách 1:

cân m g K2Cr2O7 , V =200ml và Vxd =10ml và

Na 2s2o3 =25 gam K2Cr2O7 có trong m g K2Cr2O7 là :

mK2Cr2O7 K2Cr2O7

N ng ng dung d ch K2Cr2O7

(mK2Cr2O7 K2Cr2O7)* (1000/200)

(mK2Cr2O7 K2Cr2O7)* (1000/200) * (10) = 25 * 0.1

mK2Cr2O7

Cách 2: Có th công th c

mK2Cr2O7 = mDK2Cr2O7 * (NV)Na2S2O3 * (Vdm/Vxd)

V i Vdm = 200ml Vxd= 10ml

6.20

thiosunfat 0,1100N?

Cl2 + KI = I2 + KCl

I2 + Na2S2O3 = 2NaI+ Na2S2O3

mCl2 = mDCl2 * ((NV)Na2S2O3 * (1000/25) = 3,139g