Exam # 3 Spring 2000
Chemistry 111
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1. What is the partial pressure of He(g) in a vessel containing 
a mixture of 1.00 mol He(g), 2.00 mol Ne(g), and 3.00 mol Ar(g) 
with a total pressure of 12.0 atm?

First add all the moles of gas to get 6.00 mol.  Next determine 
the fraction of the moles which are He.

                           ----  = 1/6

Last multiply the total pressure times the fraction which is He:

                        1/6 x 12 = 2 atm

2. Which of the following ideal gases has the highest kinetic 
energy at 200 K?

All gases have the SAME kinetic energy at the same temperature.

3. Which of the following ideal gases has the highest average 
particle speed at 200 K?

The energy is a function of mass and velocity.  Since the 
energies are the same (Question 2), the velocities must be 
inversely proportional to the mass.  The heavier mass moves 
slower while the lightest mass moves the fastest.

4. According to the Kinetic Theory of Gases:

    The average kinetic energy of a gas is actually determined by 
the temperature but one can extend this to recognize that IF a 
gas have an average kinetic energy of a specific value THEN it 
must be at a certain temperature.  (Reverse logic)

    The expected answer was C

5. Gases will deviate from the ideal gas law as:

The question quite probably had two correct answers BUT one was 
"more" correct.

As temperature moves to either extreme the behavior of gases 
deviates from ideal.  Thus as temperature increases gas behavior 
deviates and as temperature decreases gas behavior deviates.  
When a gas condenses to a liquid, the particles can no longer 
behave as gases, the "more" correct answer was D, the gas 
approaches condensation to the liquid state.

6. The electron configuration for a P atom is:

P is in the 3rd level and in the p sublevel section of the 
periodic table.  P has ALL preceding electron configuration 
through Ne and 3s2 3p3 beyond Ne so:

7. Light traveling at a wavelength of 557.7 nm is of a green color. What is the frequency of this light? The fact it is green is irrelevant to the calculation. The same equation serves for calculating all combinations of frequency and wavelength.
c = ln
c = 2.998 x 108 m/s
n = c/l
Plug in the values:
n = (2.998 x 108 m/s /557.7 nm
Convert nm to meters by substituting the power of ten for which the "n" stands:
n = (2.998 x 108/557.7 x 10-9 m
Finish the math:
n = 5.38 x 1014 Hz
Remember Hz means 1/s 8. The frequency of light emitted when an electron moves from n=3 to n=2 is 4.570 x 1014 Hz. What is the wavelength of this light? As above use:
c = ln
c = 2.998 x 108 m/s
This time solve the equation for l:
l = c/n
Substitute the numbers:
l = 2.998 x 108 m/s /4.570 x 1014 Hz
Complete the math:
l = 6.560 x 10-7 m
Recall wavelengths are expressed in nanometers (nm) so adjust the decimal point and power of ten to get a nano- (10-9
l = 656.0 x 10-9 m
l = 656.0 nm
9. There are four sublevel types associated with electron orbitals or elements in the current preiodic table. They are: D) s, p, d, f 10. Electrons require a certain amount of energy in order to maintain their motion and position orbiting the nucleus of an atom. Since the energy for each level and sublevel is a fixed amount chemists state the energy is: A) quantized 11. Which of the following is not an allowed set of quantum numbers? n l ml ms A) 7 0 0 1/2 Perfectly good because 7 is a maximum level, l must be from 0 to n-1 which it is, ml must range from l down to -l and spin must be either +1/2 or -1/2. B) 3 1 4 -1/2 BAD! ml is NOT l to -l C) 1 0 0 1/2 Perfectly good because 1 is a level, l must be from 0 to n-1 which it is, ml must range from l down to -l and spin must be either +1/2 or -1/2. D) 7 6 5 -1/2 Perfectly good because 7 is a maximum level, l must be from 0 to n-1 which it is, ml must range from l down to -l and spin must be either +1/2 or -1/2. While we have no elements that meet this set of numbers, the values follow all the rules and we may one day find an element with these quantum numbers. 12. What quantum numbers describe a 4s subshell? Quick and easy n is the level number so it must be 4 l describes the subshell and s is 0 The answer is D. 13. What element has the lowest first ionization energy? Again, quick and easy. The lowest ionization energies are with those elements that lose elections the easiest (or have the lowest electronegativity). Look at the periodic table and figure which element is furthest to the left and farthest down. Or alternatively, see which of the given elements has the lowest electronegativity on the table. Answer: D) Cs 14. Which of the following quantum numbers violates the Pauli Exclusion Principle? The principle is that no two electrons in a single atom can have the same set of quantum numbers. Perfectly logical since quantum numbers are used to specify which electron is being observed so if two in one atom have the same number, the entire method would fail. The bad set is answer A) 15. How many unpaired electrons does an oxygen atom have? Use Hund's rule for filling: There are 2 "s" electrons which are paired and 2 "p" electrons which are unpaired. The answer is B) 2 16. Which element has the following orbital diagram beyond Ar? 4s 3d 4p () ()()()()() ( )( )( ) This element is in the 4th level and has passed the 3d level (it's full). It has 3 electrons in the p sublevel of the 4th level so it must be: C) As 17. Moving from top to bottom in a group in the periodic table atomic radius ______________________. D) increases More levels means a larger atom. 18. The most electronegative atoms have ________________ attraction for electrons in chemical bonds. A) the strongest The way to figure out this question was to read the above sentence and fill in each choice as you read it. They were all worded so they would read correctly. 19. How many valence electrons does a Selenium (Se) atom have? Check the group number, if greater than 10 cover the 1 (subtract 10) and that's the number of valence electrons. The answer is: A) 6 20. What would the Lewis structure of a boron (B) atom look like? First see what group B is in. It is the 13th so subtract 10 an B has 3 electrons. So the Lewis structure would be: . B) .B. 21. Which of the following Lewis structures is(are) correct? + | H | | | | i) NH4+ | H - N - H | | | | | H | Looks OK from here. .. .. .. ii) CS2 :S - C - S: .. .. .. Octets look good BUT count the total number of electrons. There are 8 on each S and 4 on C for a total of 20 BUT 2 S and 1 C only have 16 so this is wrong. .. .. iii) PF3 :F - P - F: .. | .. :F: .. P does not have a complete octet. iv) SF2 .. . . .. :F - S - F: .. . . .. Every atom has a complete octet and there are exactly the right number of total electrons. The answer is: D 22. Which is a correct Lewis structure for OCN- ion? A) carbon has 5 bonds, can't happen B) carbon only has 4 electrons, doesn't meet octet rule. C) Meets octet rule BUT carbon has picked up electrons from somewhere. Carbon only has 4 electrons so something is wrong. D) Perfect. Octet rule met. Proper number of electrons. No electron shift. 23. An atom has the Lewis structure: 2- | :O: | | .. | | .. | | :O - X - O: | | .. .. | | | What is a possible element for X? A) F only forms single bonds cause it only needs one electron. If it was F, then there are not enough total electrons B) B has only 3 electrons to share. So it will only form 3 bonds and there are more total electrons than could be with B. C) C fits just right since it forms 4 bonds and the total number of electrons adds up. D) N likes 3 or 5 bonds and the total number of electrons is not right. 24. What is the molecular shape of NF3? .. . . .. :F - N - F: .. | .. :F: .. Since there are only three attached atoms, one might want to call this a planar molecule. BUT, the extra electrons repel the others. The electron shape would be predicted to be tetrahedral since there are four pairs of electrons on the central atom. It cannot fit in the tetrahedral shape because a tetrahedron by definition must have all angles equal. This molecule is therefore: D) triangular (trigonal) pyramid 25. Which of the following is linear? A) Extra electrons on the central atom repel the others and cause this molecule to be bent. B) Same as A. C) Same as A. D) This is the only one where all bonds to the central atom are the same and there are no extra electrons to repel. This is the correct answer. 26. Which of the following has a tetrahedral molecule shape? A) Nope, not equal angles to the bonds because the repulsive forces only match on three of the bonds B) Nope, not equal angles since there are two bonded atoms and two unbonded pairs. C) OK, we hit paydirt. Four matched bonds with four matched atoms. Can't do any better than this. D) See above. 27. What is the molecular shape of phosgene, CCl2O? First a note, phosgene is a toxic gas used in World War I to poison the troops. Next, look at the probable structure. C is the most likely central atom because Cl only gets one bond and O only gets two. S forms 4 bonds. Cl \ C = O / Cl Double bonds behave as single bonds in structures so this is an AX3 with no extra electrons on the central atom so the answer is: A) triangular planar 28. The structure of phosphate, PO4, ion is: 3- | :O: | | .. | .. | | :O - P - O: | | .. | .. | | :O: | | | The O-P-O bond angle is closest to: In order to answer this one, the testtaker had to first get out of the "paper" mode and look at the 3-dimensional structure. I admit this one was a bit harder than other and when one is testing it is not always easy to think clearly. The O-P-O on the question line affected some who chose D) 180. Had the question line shown O these same folks would have chosen A) 90. | P-O The correct answer is due to the fact that this is a tetrahedral molecule which has bond angles of B) 109. 29. The bond angle in SF2 is closest to: .. . . .. :F - S - F: .. . . .. Since there are extra electrons on the central atom we know this is not linear, not 180. Since there are two extra pairs, we should also know this is not a planar molecule, so not 120. That leaves 90 and 109 for answers. The closest shape this molecule approaches is that of a tetrahedron, based on electron shape. SO the angle is probably closest to 109. 30. Which molecule has a bond angle closes to 120? A) SO2 Look at Lewis structure. . . S // \ .. :O O: .. .. Sure looks like a triangular planar molecule to me. And triangular planar molecules have 120 degree angles. B) SF2 from previous question we know this one is 109. C) CO2 from class discussion and from problem 25 we know this is linear (180) D) PF3 Now this is a possible but we already have one answer. In order to check we must draw the Lewis structure. .. . . .. :F - P - F: .. | .. :F: .. To form an octet, P, the central atom, MUST have an extra pair of electrons so the molecule is not planar and thus the angles are not 120. Also note P is under N in the periodic table which means it behaves similarly to N and we known N compounds are triangular pyramid, i.e the extra electrons bend the molecule from the planar shape but not enough to be a tetrahedral.