CHEM 1211L PRINCIPLES OF CHEMISTRY I LAB

FALL 2010

MIDTERM EXAMINATION

NAME:

DATE:

Examine the following picture and answer the relevant questions

1. The distance between two lines is equivalent to _____0.1______ ( a number) mL. Therefore the volume shown by the bottom of the meniscus is ____4.55____ ( a number) ________ (units)

2. Two substances have the same weight. Which one has a lower volume? Explain.

a. One with a larger density, because in the relation between weight, volume and density, increasing the density leads to increasing the volume

b. One with a smaller density, because in the relation between weight, volume and density, decreasing the density leads to increasing the volume

c. One with a larger density, because in the relation between weight, volume and density, increasing the density leads to decreasing the volume

3. When you have a choice between the 10 ml graduated and a graduated 10 mL pipette to measure the same volume of a liquid, which tool would you rather use? Explain. Hint: Examine the distances between the lines of the two tools and compare the amounts that they represent.

a. The pipette, because fluctuations from the target line involve larger amounts of liquid and lead to a higher reading error

b. The cylinder, because fluctuations from the target line involve smaller amounts of liquid and lead to a lower reading error

c. The pipette, because fluctuations from the target line involve smaller amounts of liquid and lead to a lower reading error.

The next few questions refer to the following problem:

" An 8.47 g sample of a solid is placed in a 25.00 ml flask. The remaining volume in the flask is filled with benzene in which the solid is insoluble. The solid and benzene together weigh 24.54 g. The density of the benzene is 0.879 g/ml. What is the density of the solid?"

4. How do you find the information requested in the problem? Are all needed parameters directly available in order to find the answer? If not, what parameter is to be determined next?

a. Mass of solid = d / volume of solid. The volume of the solid is given, but not the mass, which must be determined next.

b. Density of the solid = mass of solid / volume of solid. The mass of the solid is given, but not the volume, which must be determined next.

c. Volume of the solid = mass of solid x d. The mass of the solid is given, but not the volume, which must be determined next

5. How do you find the volume of the solid? Are all needed parameters directly available in order to find the answer? If not, what parameter is to be determined next?

a. Volume of solid = total volume - volume of benzene. The total volume is given, but not the volume of benzene, which must be determined next.

b. Volume of solid = total volume - volume of benzene. The volume of benzene is given, but not the total volume, which must be determined next.

c. Volume of solid = volume of benzene - total volume. The total volume is given, but not the volume of benzene, which must be determined next.

6. How do you find the volume of benzene? Are all needed parameters directly available in order to find the answer? If not, what parameter is to be determined next?

a. volume of benzene = mass of benzene x density of benzene. The density of benzene is known, but not its mass, which must be determined next.

b. volume of benzene = density of benzene / mass of benzene. The mass of benzene is known, but not its density, which must be determined next.

c. volume of benzene = mass of benzene / density of benzene. The density of benzene is known, but not its mass, which must be determined next.

7. How do you find the mass of benzene? Are all needed parameters know in order to find the answer? Justify your answer.

a. mass of benzene = volume x density of benzene. The volume of benzene should be determined next.

b. mass of benzene = total mass - mass of solid. All parameters are known.

c. mass of benzene = mass of solid - total mass. The mass of the solid should be determined next.

8. How should you use the correct answer to the previous question? Justify your answer.

a. Plug the volume of benzene in the expression of the mass of benzene, and use the mass of benzene to figure out the volume of the solid, which in turn is used to find the requested density of the solid.

b. Plug the mass of benzene in the expression of the volume of benzene, and use the volume of benzene to figure out the volume of the solid, which in turn is used to find the requested density of the solid.

c. Plug the mass of benzene in the expression of the volume of benzene, and use the volume of benzene to figure out the density of the solid, which in turn is used to find the requested volume of the solid.

Consider the following assignment.

Using the attached graphic, determine GRAPHICALLY and MANUALLY (using a ruler and a pencil) the atmospheric pressure that corresponds your personal air density of 1.1ABC, where A, B, and C are the 3 last non-zero digits in your ABAC ID. Show on the graph how you reach your answer, and indicate what that answer is. No credit will be given for dropping the last answer alone. Calculations will not be accepted.

9. Examine the space between two consecutive lines on the axes and indicate the amount of the property they represent. Specify both the number and the corresponding unit.

On the x axis, the space between 2 lines represents:

a. __________ 1 ________ %

On the y axis, the space between 2 lines represents __________ 0.004 ________ g/mL

10. Indicate your assigned density and the corresponding atmospheric pressure

My assigned density = ___________1.1949 _________

Atmospheric pressure corresponding to my assigned density = _________ 27.7 _________ %

11. How do you determine the oxidation number of the atoms of a molecule such as Cr2(SO4)3?

a. Use the preset ON of SO4 group to figure out the ON of Cr and S

b. Use the preset ON of O and S to figure out the ON of Cr

c. Use the ON S of to figure out the ON of SO4 group Cr and

12. Determine the oxidation number of Mo and C in Mo2(CO3)3. Show how you proceed. No credit will be given for the last answer alone.

ON of CO3 = -2 ( see pg 91 in your textbook)

C + 3(-2) = -2

C – 6 = -2

C = -2 + 6 = +4

2 x Mo + 3(-2) = 0

2 x Mo – 6 = 0

2 x Mo = 0 + 6 = 6

Mo = +3

13. What is the name of As3N5? Justify your answer.

a. Arsenic nitride. It is an ionic compound and according to the rules of nomenclature the atom from the leftmost column in the periodic table is named first.

b. Nitrogen Arsenide. It is a covalent compound and according to the rules of nomenclature the atom from the rightmost column in the periodic table is named first.

c. Triarsenic pentanitride. It is a covalent compound and according to the rules of nomenclature the atom from the leftmost column in the periodic table is named first, and the repeats of each atom are shown using greek prefixes.

14. What is the structure of Manganese (IV) phosphate? Justify your answer

a. Mn3(PO4)4. It takes 3 Mn(+4) cations to neutralize 4 PO4(3-) anions

b. Mn3(PO3)4. It takes 3 Mn(4+) cations to neutralize 4 PO3(4-) anions

c. Mn4(PO4). It takes 4 Mn(4+) anions to neutralize 1 PO4(4-) cations

15. What is the name of HIO4? Justify your answer

a. Iodic acid. It is an acid made a proton and the iodide anion

b. Periodic acid. It is an acid made a proton and the periodate anion

c. Iodous acid. It is an acid made a proton and the iodite anion

16. What are the names of the following molecules?

Molecules:

A: K2HPO4

B: P2O3

C: Ga(ClO3)3

D: Co2S3

E: HCrO4

Answer Choices:

a. A = Potassium hydrogenphosphate; B = diphosphorus trioxide; C = Gallium chlorate; D: Cobalt (III) sulfide; E = Chromic Acid

b. A = Dipotassium hydrogenphosphate; B = Phosphorus Oxide; C = Gallium trichlorate; D: Cobalt (III) sulfide; E = Chromous Acid

c. A = Potassium (I) hydrogenphosphate; B = Diphosphorus trioxide; C = Gallium (III) chlorate; D: Cobalt sulfide; E = Hydrogen chromoxide

17. What are the formulas of the following molecules? Write the formulas in front of the names

Molecules:

A: Lithium phosphate: Li3PO4

B: Tricarbon Tetraphosphide : C3P4

C: Magnesium perchlorate : Mg(ClO4)2

D : Vanadium (V) nitride : V3N5

E : Nitrous acid : HNO2

18. If in the reaction between Na2CO3 and HCl you did not use enough HCl, how would that affect the mole-to-mole ratio of NaCl / Na2CO3?

1. It would decrease, because some of the collected solid would be NaCl, which would increase the denominator in the ratio.
2. It would decrease, because some of the collected solid would still be unreacted Na2CO3, the weight of which would divided by the molar weight of NaCl, which would lead to an erroneously lower number of moles of NaCl and increase the numerator in the ratio.

3. It would increase, because some of the collected solid would still be unreacted Na2CO3, the weight of which would divided by the molar weight of NaCl, which would lead to an erroneously larger number of moles of NaCl and increase the numerator in the ratio.

19. If Na2CO3 was replaced by K2CO3 in the experiment, the following reaction would take place:

K2CO3 + 2HCl -> 2KCl + H2O + CO2

Would the KCl / K2CO3 mole ratio change compared to the original mole ratio as a result of the replacement? Justify your answer.

Examine the following situation:

A student worker made a mistake and placed sodium bicarbonate (MW = 84 g/mol) instead of sodium carbonate (MW = 106 g/mol) on the benches for students to use in the experiment on determination of mole-to-mole ratio. Suppose you used the exact same weights of reagents as indicated in the lab. As a result the reaction that actually takes palce is the following:

NaHCO3 (aq)+ HCl (aq) à NaCl (aq) + H2O (l) + CO2 (g)

20. How many moles of Na2CO3 are shown by the calculations? How many moles of the carbonate reagent are actually there? How do the two numbers of moles compare?
1. #mol calculated = 0.5 g / 106 g/mol = 5.95E-2 mol; #mol actually present = 0.5 g / 84 g/mol = 4.72E-2 mol. Calculations show less moles of carbonate than there actually are

2. #mol calculated = 0.5 g / 106 g/mol = 4.72E-2 mol; #mol actually present = 0.5 g / 84 g/mol = 5.95E-2 mol. Calculations show less moles of carbonate than there actually are

3. #mol calculated = 106g / 0.5 g/mol = 21.2 mol; #mol actually present = 0.5 g / 84 g/mol = 5.95E-2 mol. Calculations show more moles of carbonate than there actually are

21. How many moles of NaCl would you expect to find in the calculations? How many moles of the product can you actually get?
1. #mol of NaCl expected = 2 x #mol of Na2CO3 calculated = 2 x 4.72E-2 mol = 9.44E-2 mol, because of the 2/1 NaCl / Na2CO3 mole ratio in the equation of the expected reaction; #mol actually possible = same as #mol of product actually present = 5.95E-2 mol, because of the 1/1 NaCl / NaHCO3 mole ratio in the equation of the actual reaction.

2. #mol of NaCl expected = same as #mol of Na2CO3 calculated = 4.72E-2 mol = 9.44E-2 mol, because of the 1/1 NaCl / Na2CO3 mole ratio in the expected reaction; #mol actually possible = 2 x #mol of product actually present = 5.95E-2 mol, because of the 2/1 NaCl / NaHCO3 mole ratio in the actual reaction.

3. #mol of NaCl expected = 1/2 x #mol of Na2CO3 calculated = 1/2 x 4.72E-2 mol = 2.36 E-2 mol, because of the 2/1 NaCl / Na2CO3 mole ratio in the expected reaction; #mol actually possible = same as #mol of product actually present (1/1 mole ratio) = 5.95E-2 mol, because of the 1/1 NaCl / NaHCO3 mole ratio in the actual reaction.

22. How does the actual NaCl / Na2CO3 mol ratio compare to the expected one? Justify your answer.
1. Actual mol ratio = 9.44 E-2 mol NaCl expected / 5.95E-2 mol Na2CO3 calculated = 1.58. Expected mole ratio = Theoretical mole ratio from equation = 2. The actual mole ratio is higher the expected one.

2. Actual mol ratio = 9.44 E-2 mol NaCl expected / 4.72E-2 mol Na2CO3 calculated = 2. Expected mole ratio = Theoretical mole ratio from equation = 2. The actual mole ratio is the same as the expected one.

3. Actual mol ratio = 5.95E-2 mol NaCl possible / 4.72E-2 mol Na2CO3 calculated = 1.26. Expected mole ratio = Theoretical mole ratio from equation = 2. The actual mole ratio is lower than the expected one.

23. What problem are you likely to encounter in your results if the product of reaction of Mg with O2 contains Mg3N2 resulting from the reaction of Mg with N2 present in the air? Explain.

a. The empirical formula will show an artificial excess of O, because the calculated weight of O will be higher than expected.

b. The empirical formula will show an artificial excess of Mg, because the calculated weight of O will be lower than expected.

c. The empirical formula will show an artificial excess of Mg, because the calculated weight of O will be higher than expected.

Consider the following problem.

“Determine the empirical formula of a compound made of 87.5 g of N and 12.5 g of H.”

24. How should you use the information provided in order to find the requested empirical formula? Explain

a. Use the weights as subscripts in the raw formula leading to the empirical formula of the molecule.

b. Use the weights to calculate the number of moles of N and H, because they are used as subscripts in the raw formula leading to the empirical formula of the molecule.

c. Use the weights to calculate the molarity of N and H, because it is used as subscripts in the raw formula leading to the empirical formula of the molecule.

25. Calculate the number of moles of N and H. Show how you proceed.

a. #mol of N = 87.5 g x 14.0 g/mol = 1225 mol. #mol of H = 12.5 g x 1.01 g/mol = 12.6 mol.

b. #mol of N = 87.5 g / 14.0 g/mol = 6.25 mol. #mol of H = 12.5 g / 1.01 g/mol = 12.4 mol.

c. #mol of N = 14.0 g/mol / 87.5 g = 0.16 mol. #mol of H = 1.01 g/mol / 12.5 g = 0.0808 mol.

26. Determine the raw and the empirical formula for the compound.

a. Raw Formula: N6.25H12.4 Empirical formula: N6H12, obtained by rounding the subscripts to the closest whole number

b. Raw Formula: N12.4H6.25 Empirical formula: N2H, obtained by dividing all subscripts by the smallest subscript (6.25)

c. Raw Formula: N6.25H12.4 Empirical formula: NH2, obtained by dividing all subscripts by the smallest subscript (6.25)

27. Bonus Question. Solve the problem at the end of question 3. Show your calculations, both on numbers and units (90 % of the grade).