Name:   Professor Key                                                                         

Chemistry 3600

First Examination

October 6, 1999

 

Directions: There are 4 questions, each worth equal points.  Some are more difficult than others.  Read over all questions before you start to answer.  Answering the harder questions last will insure that you obtain the maximum score.  You must show your work and pertinent formula's to get full credit.  The final number answer is only ten percent of the total score.

 

Statistics formulas:

 

Useful data:

 

Values of Q @ 90% confidence

Q 0.94 0.74 0.64 0.56 0.51 0.47 0.44 0.41
N 3 4 5 6 7 8 9 10

 

Values of Student's t @ 95% confidence

t

12.71

4.303

3.182

2.776

2.571

2.447

2.365

2.306

2.262

2.228

d.f.

1

2

3

4

5

6

7

8

9

10

 

Values of F @ 95% confidence

d.f. - s2

d.f. - s1

 

2

3

4

5

6

2

19.00

19.16

19.25

19.30

19.33

3

9.55

9.28

9.12

9.01

8.94

4

6.94

6.59

6.39

6.26

6.16

5

5.79

5.41

5.19

5.05

4.95

6

5.14

4.76

4.53

4.39

4.28


1.    Analysis, Concentrations, and Calibration

A)    List and give examples of the 4 types of reactions that may be used for titrimetric analysis

 

1 - Acid/Base

 

2 - Redox

 

3 - Precipitation

 

4 - Complex-forming

 

B)    A solution is prepared by dissolving 9.88 g trichloroacetic acid (F.W. 163.39) in 500 mL solution. At this concentration, the acid is 70% dissociated. What is the formality of trichloroacetic acid?

 

 

       What are the molarities of trichloroacetic acid and trichloroacetate?

 

 

C)    Write definitions or give formulas to describe the following terms;

Standard: An object, sample, or procedure used to calibrate a method.

 

Determinant Error: An error arising from known sources.

 

Analyte: Substance to be determined in a chemical analysis.

 

Titrant: Reagent solution used in volumetric or titrimetric analysis. 

 

Parts-per-Million: Fraction expressed in units per 106 units.


2. Statistics

A)    Use the Q test to determine if the largest value can be dropped from the following data set

0.529, 0.765, 0.439, 0.530, 0.565, 0.521

 

 

Since Qcalc>Qtable, datum may be rejected from the set.

 

B)    Report the mean, standard deviation, and relative standard deviation based on the following data set. Be sure to show your work.

24.90, 24.98, 25.07, 24.88, 25.17

 

 

 

C)    Calculate the confidence interval and report the statistically probable mean value at the 95% confidence level

 

 

D)    Apply the F- and t- tests to determine if there is a significant difference in following data sets (@ 95% confidence). [x1=42.34, s1=0.15, n1=6] [x2=43.24, s2=0.10, n2=4]

- Do the data sets differ in their standard deviations?

 

 

Since Fcalc<Ftable, the S.D. are not different.

- Are their means different?

 

 

Since tcalc>ttable for n1+n2-2 degrees of freedom (ttable=2.306), means are different!

 


3. Titrimetric Analysis

A 3.2306 g iron ore sample is dissolved in acid. The iron is reduced to Fe2+ and the solution is diluted to 250 mL in a volumetric flask. 50 mL aliquots are titrated with 0.01753 F standardized solution of K2Cr2O7. The average titrant volume was 36.30 mL to reach the equivalence point. The redox reaction is

 

A)    Calculate the weight percent iron (AW 55.847) in the ore sample.

 

Each aliquot has

The weight percent is

 

B)    Express the weight percent in terms of Fe2O3 (FW 159.69).


4. Gravimetric Analysis

A 0.5664 g ore sample is dissolved in nitric acid and then filtered. The aluminum is present in solution as Al3+. The solution is made basic with ammonium hydroxide, NH4OH, and the aluminum hydroxide, Al(OH)3 (FW 78.004), precipitates. This gel is filtered in a porous glass crucible, rinsed with dilute ammonium hydroxide, ignited, cooled in a desiccator, and weighed. The resulting alumina, Al2O3 (FW 101.94), weighed 0.1605 g.

 

A)    Answer the following questions;

1-    Why is the solution filtered after acid dissolution?

 

To remove insoluble matrix material (like sand, etc.)

 

2-    Why rinse with ammonium hydroxide solution?

 

To avoid peptization or loss of precipitate. Ammonium hydroxide is a volatile elcctrolyte.

 

3-    What chemical transformation takes place during ignition? (Show the balanced chemical equation)

 

2Al(OH)3 -> Al2O3 + 3H2O

 

4-    Why use a desiccator during cooling?

 

To avoid weight gain by adsorption of atmospheric water

 

B)    Calculate the weight percent Al (AW 26.9815) in the sample.