PLTL Workshop – Assessment (2) CHEM 111

Chemistry 111

Final Assessment

T he following assessment is based on the workshops you have encountered this semester. To better develop the materials and to make sure that you and future students are well-instructed, we have developed this set of follow-up exercises to be done WITHOUT the assistance of the peer leader or the other members of your group. Though this assessment is being done on an individual basis, you will not be graded on these exercise but should instead use this exercise to determine how much you really understood from your earlier workshops.

Also, we have included two assessment surveys for you to give feedback about the quality of your instruction, your peer-to-peer interactions, your relationship with your peer leader as well as your opinions about your progress in the overall course. Though your peer leaders will go over your answers to the exercises, your instructors and peer leaders will NOT have access to your surveys/comments, so please feel free to be as forthcoming as possible.

Chemical Reactions

Chemical reactions have been known and applied for many years. For example, ancient people knew that if carbon (charcoal) was mixed with iron oxides and heated, elemental iron and carbon dioxide would be formed. This practical knowledge was attained without any concept of atoms, molecules and reactions. By the nineteenth century, the study of stoichiometry allowed chemists to determine masses of reactants and products during the reaction. We will use stoichiometry to allow us to predict the mass of a reactant or product using a balanced equation.

Balanced Equations

Chemical equations may take many forms. The statement above that iron oxide reacts with

charcoal to produce elemental iron and carbon dioxide is one form of describing a chemical

reaction. However, this type of statement does not allow any quantitative information to be

determined. Using the information above, and a little knowledge of chemical formulas, we can

produce a balanced chemical equation to use in quantitative determinations. The term “iron

oxide” is ambiguous because iron and oxygen may react with several different ratios. For our

purposes, we will assume that the iron oxide in question is iron(III) oxide. The formula of

iron(III) oxide is Fe2O3. Carbon is a monatomic element with the symbol C. Elemental iron is

Fe. Carbon dioxide has the formula CO2. We can write an unbalanced equation as:

Fe2O3 + C → Fe + CO2

In order to get quantitative information from this equation it must be balanced. Balancing the

equation would result in the following equation:

2Fe2O3+ 3C → 4Fe + 3CO2

This equation tells us that 2 moles of Fe2O3 will react with 3 moles of C to produce 4 moles of Fe and 3 moles of CO2. We will use this information to form conversion factors to allow us to

calculate the amount of one substance from the amount of another substance.

Molar Masses

An additional conversion factor necessary in stoichiometric calculations is molar masses. As we learned in the last workshop, the atomic masses from the periodic table expressed in grams

represent Avogadro’s number of molecules. One mole is defined to be Avagadro’s number of

particles. If the atomic mass of an element is expressed in grams, this represents one mole of

atoms.

1 mol = 6.022x1023 atoms = atomic mass in grams

Similarly, the molar mass of substances is defined to be the mass of one mole of substance. For

compounds, calculation of the molar mass is accomplished by adding the atomic masses. If the

molar mass is expressed in grams, this represents one mole of the compound.

1 mol = 6.022x1023 formula units = molar mass in grams

We use the more general term “formula units” instead of molecules because ionic compounds do not exist as molecules.

Factor Unit Conversions and Calculation of Moles

We can use the conversion factors above to calculate the number of moles of a substance from the mass of the substance.

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Self Test

1. 1. A balloon is filled with helium gas at 27°C and 1.00 atm pressure. As the balloon rises, the volume of the balloon increases by a factor of 1.60 and the temperature decreases to 15°C. Assuming that no helium escapes from the balloon, what is the final pressure?

2. Pyrophosphoric acid is composed of 2.27% hydrogen, 34.80% phosphorus, and the remainder is oxygen.

a) What is the mass of each element present in a 125.0 g sample? What is the number of moles present in the sample?

b) Find the simplest whole-number ratio of the number of moles of each element. This can be found by dividing the number of moles of each element by the number of moles of the element with the smallest number of moles. Your result gives the empirical formula of pyrophosphoric acid.

c) The molar mass of pyrophosphoric acid is 177.97 g/mol. What is its molecular formula?

3. Determine the freezing and boiling points of a solution made by dissolving 30.0 g of sucrose, C12H22O11, in 100.0 g of water.

4. Fill in the chart below, using a periodic table as the only source of additional information.

Compound / Empirical Formula / Molar Mass / Moles in 100.0 g
H2O
H2O2
BaSO4

5. Calculate the percentage composition by mass of each of the compounds in Question 1. The first compound has been completed as an example.

Compound / Percentage Composition by Mass
H2O / 88.8% O, 11.2% H
H2O2
BaSO4


Recitation Day and Time ______

For each item, please circle the number that corresponds to your response:

1= strongly agree; 2= agree; 3= disagree; 4= strongly disagree

The Workshops are closely related to the material taught in the lectures.
I believe that the Workshops have improved my grade.
Interacting with the other group members increases my understanding.
The materials in the workshop were useful for group work.
Students who are uninterested or unmotivated made it difficult for others to benefit from the workshops.
I felt comfortable with the Workshop leader.
I would like to be a Workshop leader in the future. / 1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4

On average, I spend the following number of hours per week studying (in addition to time spent at lectures and Workshops):

a. 0-2 hours b. 2-4 hours c. 4-6 hours d. 6-8 hours e. 8-10 hours

Please rate each of the following activities according to the amount of Workshop time devoted to the specified activity. Use the following scale:

1= most of the time; 2= a moderate amount of time; 3= a small amount of time; 4= almost no time

The self test
Group activities
Small group problem solving
Independent problem solving
Laboratory connections / 1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4

What was the last topic that you covered in lecture?

I. Workshop materials

Did the workshops help learn the course material?

Did the group activities increase your understanding?

What part of the workshop materials was the most helpful? (Background information, self-test, group problems, laboratory connections)

What part was the least helpful?

Are the problems too hard, too easy, or about right?

II. Role of the leader

What was the role of the leader in the group?

Was the workshop leader well prepared?

Did the workshops improve slightly, greatly or not at all since the first assessment?
III. Group Interactions

How did you spend most of your time during the workshops?

Did you feel comfortable interacting with the group?

Did you learn from the other members in the groups?

What was the best part of the workshop?

What was the worst aspect of the workshop?

Do you feel confident that you will successfully complete Chemistry 111 with a passing grade (C or better)?

IV. General observations and comments.

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