For parts of the free-response question that require calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and equations may be included in your answers where appropriate.
A sample of a pure element is analyzed using a mass spectrometer. The results are shown below.
The figure presents a bar graph. The horizontal axis is labeled Atomic Mass, in a m u, and the numbers 24, 25, and 26, are indicated. The vertical axis is labeled Percent Abundance, and the numbers 0 through 100, in increments of 20, are indicated. There are 3 bars of data. The data presented by the bars are as follows. Note that all values are approximate. Bar 1, 24 a m u, 79 percent abundance. Bar 2, 25 a m u, 10 percent abundance. Bar 3, 26 a m u, 11 percent abundance.
(a) How many different isotopes of the element were in the sample?
(b) Describe how to use the information from the mass spectrum to determine the average atomic mass of the element.
(c) Identify the element.
(d) Write the ground-state electron configuration of an atom of the element that you identified in part (c).
Answers
Answer:
Throughout the exam the following symbols have the definitions specified unless otherwise noted.
L, mL = liter(s), milliliter(s)
g = gram(s)
nm = nanometer(s)
atm = atmosphere(s)
mm Hg = millimeters of mercury
J, kJ = joule(s), kilojoule(s)
V = volt(s)
mol = mole(s)
ATOMIC STRUCTURE
E = energy
ν = frequency
λ = wavelength
Planck’s constant, h = 6.626 × 10−34 J s
Speed of light, c = 2.998 × 108
ms−1
Avogadro’s number = 6.022 × 1023 mol−1
Electron charge, e = −1.602 × 10 19 coulomb −
E = hν
c = λν
EQUILIBRIUM
Equilibrium Constants
Kc (molar concentrations)
Kp (gas pressures)
Ka (weak acid)
Kb (weak base)
Kw (water)
Kc = [C] [D]
[A] [B]
c d
a b , where a A + b B R c C + d D
Kp = C D
A B
( )( )
( )( )
c d
a b
P P
P P
Ka = [H ][A ]
[HA]
Kb = [OH ][HB ]
[B]
Kw = [H+][OH−] = 1.0 × 10−14 at 25°C
= Ka × Kb
pH = −log[H+] , pOH = −log[OH−]
14 = pH + pOH
pH = pKa + log [A ]
[HA]
pKa = −logKa , pKb = −logKb
KINETICS
k = rate constant
t = time
t
½ = half-life
Directions: Questions 1–3 are long free-response questions that require about 23 minutes each to answer and are
worth 10 points each. Questions 4–7 are short free-response questions that require about 9 minutes each to answer
and are worth 4 points each.
Write your response in the space provided following each question. Examples and equations may be included in
your responses where appropriate. For calculations, clearly show the method used and the steps involved in arriving
at your answers. You must show your work to receive credit for your answer. Pay attention to significant figures.
1. The compound urea, H2NCONH2 , is widely used in chemical fertilizers. The complete Lewis electron-dot
diagram for the urea molecule is shown above.
(a) Identify the hybridization of the valence orbitals of the carbon atom in the urea molecule.
(b) Urea has a high solubility in water, due in part to its ability to form hydrogen bonds. A urea molecule and
four water molecules are represented in the box below. Draw ONE dashed line (----) to indicate a possible
location of a hydrogen bond between a water molecule and the urea molecule.
H2NCONH2(s) ←→ H2NCONH2(aq)
The dissolution of urea is represented by the equation above. A student determines that 5.39 grams of
H2NCONH2 (molar mass 60.06 g/mol) can dissolve in water to make 5.00 mL of a saturated solution at 20.°C.