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CHM 112
Section 1

Homework Assignment #16
Due March 8, 2006 by 8:05 a.m.

Practice (not to be turned in): Problems 15.61

Find the equilibrium concentrations of oxalic acid, oxalate ion, hydrogen oxalate ion, and the pH of a solution of oxalic acid: the initial molar concentration of the oxalic acid in the solution is the eighth and ninth digits of your Student ID Number divided by 1000.

Answers:

For the first step:

 

H2C2O4(aq)

+

H2O(l)

H3O+(aq)

+

HC2O4(aq)

 

Initial

Ci

 

 

 

0

 

0

Change

– x

 

 

 

+ x

 

+ x

Equilibrium

Ci – x

 

 

 

x

 

x

There is no approximation allowed in this step, so the solution requires use of the quadratic equation to solve the equation:

x2 + Ka1x – Ka1Ci = 0.

The positive solution to this gives the initial concentrations used in the second step, let this be Csec

For the second step:

 

HC2O4(aq)

+

H2O(l)

H3O+(aq)

+

C2O42–(aq)

 

Initial

Csec

 

 

 

Csec

 

0

Change

– x

 

 

 

+ x

 

+ x

Equilibrium

Csec – x

 

 

 

Csec + x

 

x

In this case, approximation is allowed so:

[hydrogen oxalate ion] = [HC2O4] = Csec

[H3O+] = Csec

[oxalate ion] = [C2O42–] = Ka2

pH = –log[H3O+]

Specific answers:

Student ID Number

Ci

M

Csec

M

[H2C2O4]e

M

[HC2O4-]e

M

[C2O42-]e

M

pH

100006330

0.030

2.1×10–2

9.×10–3

2.1×10–2

5.3×10–5

1.67

100008852

0.052

3.2×10–2

2.×10–2

3.2×10–2

5.3×10–5

1.49

100009388

0.088

4.7×10–2

4.×10–2

4.7×10–2

5.3×10–5

1.33

100010785

0.085

4.6×10–2

4.×10–2

4.6×10–2

5.3×10–5

1.34

100017560

0.060

3.6×10–2

2.×10–2

3.6×10–2

5.3×10–5

1.44

100046288

0.088

4.7×10–2

4.×10–2

4.7×10–2

5.3×10–5

1.33

100064823

0.023

1.7×10–2

6.×10–3

1.7×10–2

5.3×10–5

1.76

100077590

0.090

4.8×10–2

4.×10–2

4.8×10–2

5.3×10–5

1.32

100105142

0.042

2.8×10–2

1.×10–2

2.8×10–2

5.3×10–5

1.56

100143544

0.044

2.9×10–2

2.×10–2

2.9×10–2

5.3×10–5

1.54

100148294

0.094

4.9×10–2

4.×10–2

4.9×10–2

5.3×10–5

1.31

100160251

0.051

3.2×10–2

2.×10–2

3.2×10–2

5.3×10–5

1.49

100164496

0.096

5.0×10–2

5.×10–2

5.0×10–2

5.3×10–5

1.30

100168422

0.022

1.7×10–2

5.×10–3

1.7×10–2

5.3×10–5

1.78

100170841

0.041

2.7×10–2

1.×10–2

2.7×10–2

5.3×10–5

1.56

100173015

0.015

1.2×10–2

3.×10–3

1.2×10–2

5.3×10–5

1.91

100173826

0.026

1.9×10–2

7.×10–3

1.9×10–2

5.3×10–5

1.72

100173947

0.047

3.0×10–2

2.×10–2

3.0×10–2

5.3×10–5

1.52

100175807

0.007

6.3×10–3

7.×10–4

6.3×10–3

5.3×10–5

2.20

100176275

0.075

4.2×10–2

3.×10–2

4.2×10–2

5.3×10–5

1.38

100180084

0.084

4.6×10–2

4.×10–2

4.6×10–2

5.3×10–5

1.34

100180208

0.008

7.1×10–3

9.×10–4

7.1×10–3

5.3×10–5

2.15

100187549

0.049

3.1×10–2

2.×10–2

3.1×10–2

5.3×10–5

1.51

100188349

0.049

3.1×10–2

2.×10–2

3.1×10–2

5.3×10–5

1.51

100192352

0.052

3.2×10–2

2.×10–2

3.2×10–2

5.3×10–5

1.49

100193608

0.008

7.1×10–3

9.×10–4

7.1×10–3

5.3×10–5

2.15

100193957

0.057

3.5×10–2

2.×10–2

3.5×10–2

5.3×10–5

1.46

100194186

0.086

4.6×10–2

4.×10–2

4.6×10–2

5.3×10–5

1.33

100194417

0.017

1.4×10–2

3.×10–3

1.4×10–2

5.3×10–5

1.87

100195646

0.046

3.0×10–2

2.×10–2

3.0×10–2

5.3×10–5

1.53

100196467

0.067

3.9×10–2

3.×10–2

3.9×10–2

5.3×10–5

1.41

100197351

0.051

3.2×10–2

2.×10–2

3.2×10–2

5.3×10–5

1.49

100197396

0.096

5.0×10–2

5.×10–2

5.0×10–2

5.3×10–5

1.30

100200053

0.053

3.3×10–2

2.×10–2

3.3×10–2

5.3×10–5

1.48

100201157

0.057

3.5×10–2

2.×10–2

3.5×10–2

5.3×10–5

1.46

100203271

0.071

4.1×10–2

3.×10–2

4.1×10–2

5.3×10–5

1.39

100205446

0.046

3.0×10–2

2.×10–2

3.0×10–2

5.3×10–5

1.53

100205601

0.001

9.8×10–4

2.×10–5

9.8×10–4

5.3×10–5

3.01

100207237

0.037

2.5×10–2

1.×10–2

2.5×10–2

5.3×10–5

1.60

100210253

0.053

3.3×10–2

2.×10–2

3.3×10–2

5.3×10–5

1.48

100210303

0.003

2.8×10–3

2.×10–4

2.8×10–3

5.3×10–5

2.55

100212014

0.014

1.2×10–2

2.×10–3

1.2×10–2

5.3×10–5

1.94

100212080

0.080

4.4×10–2

4.×10–2

4.4×10–2

5.3×10–5

1.36

100212092

0.092

4.8×10–2

4.×10–2

4.8×10–2

5.3×10–5

1.31

100212152

0.052

3.2×10–2

2.×10–2

3.2×10–2

5.3×10–5

1.49

100212510

0.010

8.6×10–3

1.×10–3

8.6×10–3

5.3×10–5

2.06

100213402

0.002

1.9×10–3

7.×10–5

1.9×10–3

5.3×10–5

2.71

100214888

0.088

4.7×10–2

4.×10–2

4.7×10–2

5.3×10–5

1.33

100215062

0.062

3.7×10–2

3.×10–2

3.7×10–2

5.3×10–5

1.43

100215125

0.025

1.9×10–2

6.×10–3

1.9×10–2

5.3×10–5

1.73

100215264

0.064

3.8×10–2

3.×10–2

3.8×10–2

5.3×10–5

1.42

100215471

0.071

4.1×10–2

3.×10–2

4.1×10–2

5.3×10–5

1.39

100216695

0.095

5.0×10–2

5.×10–2

5.0×10–2

5.3×10–5

1.31

100216822

0.022

1.7×10–2

5.×10–3

1.7×10–2

5.3×10–5

1.78

100217841

0.041

2.7×10–2

1.×10–2

2.7×10–2

5.3×10–5

1.56

100218709

0.009

7.9×10–3

1.×10–3

7.9×10–3

5.3×10–5

2.10

100218865

0.065

3.8×10–2

3.×10–2

3.8×10–2

5.3×10–5

1.42

100218867

0.067

3.9×10–2

3.×10–2

3.9×10–2

5.3×10–5

1.41

100218981

0.081

4.4×10–2

4.×10–2

4.4×10–2

5.3×10–5

1.35

100219014

0.014

1.2×10–2

2.×10–3

1.2×10–2

5.3×10–5

1.94

100219019

0.019

1.5×10–2

4.×10–3

1.5×10–2

5.3×10–5

1.83

100219138

0.038

2.6×10–2

1.×10–2

2.6×10–2

5.3×10–5

1.59

100219323

0.023

1.7×10–2

6.×10–3

1.7×10–2

5.3×10–5

1.76

100219636

0.036

2.5×10–2

1.×10–2

2.5×10–2

5.3×10–5

1.61

100219715

0.015

1.2×10–2

3.×10–3

1.2×10–2

5.3×10–5

1.91

100219950

0.050

3.2×10–2

2.×10–2

3.2×10–2

5.3×10–5

1.50

100220082

0.082

4.5×10–2

4.×10–2

4.5×10–2

5.3×10–5

1.35

100220712

0.012

1.0×10–2

2.×10–3

1.0×10–2

5.3×10–5

2.00

100220848

0.048

3.1×10–2

2.×10–2

3.1×10–2

5.3×10–5

1.51

100220947

0.047

3.0×10–2

2.×10–2

3.0×10–2

5.3×10–5

1.52

100221065

0.065

3.8×10–2

3.×10–2

3.8×10–2

5.3×10–5

1.42

100221836

0.036

2.5×10–2

1.×10–2

2.5×10–2

5.3×10–5

1.61

100222238

0.038

2.6×10–2

1.×10–2

2.6×10–2

5.3×10–5

1.59

100222876

0.076

4.3×10–2

3.×10–2

4.3×10–2

5.3×10–5

1.37

100223382

0.082

4.5×10–2

4.×10–2

4.5×10–2

5.3×10–5

1.35

100223879

0.079

4.4×10–2

4.×10–2

4.4×10–2

5.3×10–5

1.36

100224279

0.079

4.4×10–2

4.×10–2

4.4×10–2

5.3×10–5

1.36

100224930

0.030

2.1×10–2

9.×10–3

2.1×10–2

5.3×10–5

1.67

100226239

0.039

2.6×10–2

1.×10–2

2.6×10–2

5.3×10–5

1.58

100229080

0.080

4.4×10–2

4.×10–2

4.4×10–2

5.3×10–5

1.36

100229858

0.058

3.5×10–2

2.×10–2

3.5×10–2

5.3×10–5

1.45

100243524

0.024

1.8×10–2

6.×10–3

1.8×10–2

5.3×10–5

1.74