Chem 30A Midterm Exam

Oct 28, 2005

Avogadro’s number = one mole = 6.022 x 1023 of something.

Planck’s constant: h = 6.626 x 10-34 J-s

Speed of light: c = 3.00 x 108 m/s

For light: Ephoton= hυ and λυ=c

1.  Upon eating, your stomach secretes hydrochloric acid. Taking TUMS (calcium carbonate or CaCO3) neutralizes the HCl and relieves heartburn.

a.  Write a balanced chemical equation for this reaction assuming the products are CaCl2, CO2, and H2O.

b.  Write the complete ionic equation.

c.  Write the net ionic equation.

2.  The frequency of the radiation used in all microwave ovens sold in the United States is 2.45 GHz (The unit GHz stands for “gigahertz”: 1 GHz is 1 billion cycles per second, or 109 Hz, or 109 s-1).

a.  What is the wavelength, λ, in meters of this radiation? How does this compare to the wavelength of typical visible red light, say with λ = 650 nm?

b.  What is the energy of a photon of the above microwave radiation? How does this compare with the energy of a photon of red light with λ = 650 nm?

3.  The smell of the sea comes from a gas called dimethyl sulfide, (CH3)2S or C2H6S produced by dying phytoplankton, tiny plants that are the base of the ocean’s food chain.

a.  What is the molecular mass of dimethyl sulfide to at least four signifigant figures?

b.  How many moles of dimethyl sulfide are in a 1.0 x 10-6g (1.0 μg or 1.0 microgram) sample?

c.  How many molecules are in the sample?

d.  How many carbon atoms are in the sample?

4.  Gasoline is a mixture of many hydrocarbons. One of these hydrocarbons is nonane, C9H20.

a.  Write a balanced equation for the complete burning of nonane.

b.  Give the microscopic or particulate interpretation of the chemical equation in words.

c.  Give the macroscopic interpretation of the chemical equation in words.

5.  One set of reactants for rocket fuel is hydrazine, N2H4, and hydrogen peroxide, H2O2 which react spontaneously and vigorously when mixed: N2H4 (l) + 2H2O2 (l) à N2 (g) + 4H2O (g). A chemist tests a rocket motor by mixing 0.50 mol N2H4 with 0.90 mol H2O2.

a.  What is the limiting reactant?

b.  How many moles of water will be obtained?

c.  What is the excess reactant?

d.  How much of the excess reactant is left over in moles? in grams?

5.  One of the space shuttle rocket engines uses a reaction of pure hydrogen with pure oxygen to give hot gaseous water: 2H2 + O2 à 2H2O.

a.  Suppose you have 1.00 kg each of hydrogen and oxygen. What is the limiting reactant? And how many kg of water will be produced in this case?

b.  If you want to fill the shuttle tanks without carrying any unused reactant as excess weight, how many kg of O2 are needed for each kg of H2?

6.  Balance each of the following reactions. Write complete and net ionic equations where appropriate. Then classify the reactions in as many ways as possible: precipitation, redox, synthesis, combustion, decomposition, acid base neutralization, etc.

a.  Na(s) + H2O(l) à NaOH(aq) + H2(g)

b.  C8H18(l) + O2(g) à CO2(g) + H2O(l)

c.  Sr(OH)2(aq) + HNO3(aq) à Sr(NO3)2(aq) + H2O(l)

d.  Na2SO4(aq) + Ba(NO3)2(aq) à BaSO4(s) + NaNO3(aq)

e.  Na(s) + Cl2(g) à NaCl(s)

7.  Extra: Draw relative energy level diagrams for

a.  a one electron atom (through n=4, l=1) b. a multi-electronic atom (through n=4, l=1)

c.  What does the Pauli Exclusion Principle say?

d.  What does Hund’s rule say?

e.  In the above diagrams use the above rules and the Aufbau principle to populate the appropriate orbitals (or energy states) with the correct number of electrons to depict 1) ground state hydrogen on the left, and 2) ground state manganese, Mn, on the right.

f.  Write the electron configuration for manganese.

g.  What is the big difference between the energy level diagram for hydrogen and that for any multi-electronic atom?