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CHM 112
Section 1

Homework Assignment #18
Due March 24, 2006 by 8:05 a.m.

Practice (not to be turned in): Problems 15.73, 15.80, 15.81

A solution is prepared using potassium dihydrogen phosphate and potassium hydrogen phosphate. The concentration of the potassium dihydrogen phosphate equals the eighth and ninth digits of your Student ID Number divided by 100. The concentration of the potassium hydrogen phosphate equals the first two digits of your Student ID Number divided by 100. Find the pH of the solution.




Write the reactions, write the mass action expression, set up a Table of Concentrations, then solve.
     
KH2PO4(aq)
K+(aq)
+
H2PO4(aq)
     
K2HPO4(aq)
2 K+(aq)
+
HPO42–(aq)
 
H2PO4(aq)
+
H2O(l)
H3O+(aq)
+
HPO42–(aq)
 

Ka = [H3O+]e[HPO42– ]e/[H2PO4 ]e = 6.3×10–8

Initial
[H2PO4 ]init
     
0
 
[HPO42– ]init
Change
–x
     
+ x
 
+ x
Equilibrium
[H2PO4 ]init – x
     
x
 
[HPO42– ]init + x

Approximation is justified in all cases.

6.3×10–8 = x[HPO4 ]init/[H2PO42– ]init

x = [H3O+]e = (6.3×10–8)([H2PO4 ]init)/([HPO42– ]init)

The pH is found from pH = –log[H3O+].


Specific answers:


Student ID Number

[H2PO4 ]init

M

[HPO42– ]init

M

pH

100006330

0.30

0.10

6.72

100008852

0.52

0.10

6.48

100009388

0.88

0.10

6.26

100010785

0.85

0.10

6.27

100017560

0.60

0.10

6.42

100046288

0.88

0.10

6.26

100064823

0.23

0.10

6.84

100077590

0.90

0.10

6.25

100105142

0.42

0.10

6.58

100143544

0.44

0.10

6.56

100148294

0.94

0.10

6.23

100160251

0.51

0.10

6.49

100164496

0.96

0.10

6.22

100168422

0.22

0.10

6.86

100170841

0.41

0.10

6.59

100173015

0.15

0.10

7.02

100173826

0.26

0.10

6.79

100173947

0.47

0.10

6.53

100175807

0.07

0.10

7.4

100176275

0.75

0.10

6.33

100180084

0.84

0.10

6.28

100180208

0.08

0.10

7.3

100187549

0.49

0.10

6.51

100188349

0.49

0.10

6.51

100192352

0.52

0.10

6.48

100193608

0.08

0.10

7.3

100193957

0.57

0.10

6.44

100194186

0.86

0.10

6.27

100194417

0.17

0.10

6.97

100195646

0.46

0.10

6.54

100196467

0.67

0.10

6.37

100197351

0.51

0.10

6.49

100197396

0.96

0.10

6.22

100200053

0.53

0.10

6.48

100201157

0.57

0.10

6.44

100203271

0.71

0.10

6.35

100205446

0.46

0.10

6.54

100205601

0.01

0.10

8.2

100207237

0.37

0.10

6.63

100210253

0.53

0.10

6.48

100210303

0.03

0.10

7.7

100212014

0.14

0.10

7.05

100212080

0.80

0.10

6.30

100212092

0.92

0.10

6.24

100212152

0.52

0.10

6.48

100212510

0.10

0.10

7.20

100213402

0.02

0.10

7.9

100214888

0.88

0.10

6.26

100215062

0.62

0.10

6.41

100215125

0.25

0.10

6.80

100215264

0.64

0.10

6.39

100215471

0.71

0.10

6.35

100216695

0.95

0.10

6.22

100216822

0.22

0.10

6.86

100217841

0.41

0.10

6.59

100218709

0.09

0.10

7.3

100218865

0.65

0.10

6.39

100218867

0.67

0.10

6.37

100218981

0.81

0.10

6.29

100219014

0.14

0.10

7.05

100219019

0.19

0.10

6.92

100219138

0.38

0.10

6.62

100219323

0.23

0.10

6.84

100219636

0.36

0.10

6.64

100219715

0.15

0.10

7.02

100219950

0.50

0.10

6.50

100220082

0.82

0.10

6.29

100220712

0.12

0.10

7.12

100220848

0.48

0.10

6.52

100220947

0.47

0.10

6.53

100221065

0.65

0.10

6.39

100221836

0.36

0.10

6.64

100222238

0.38

0.10

6.62

100222876

0.76

0.10

6.32

100223382

0.82

0.10

6.29

100223879

0.79

0.10

6.30

100224279

0.79

0.10

6.30

100224930

0.30

0.10

6.72

100226239

0.39

0.10

6.61

100229080

0.80

0.10

6.30

100229858

0.58

0.10

6.44

100243524

0.24

0.10

6.82