1. Good morning! An average cup (250 mL) of coffee contains 125 mg caffeine (C₈H₁₀N₄O₂). What is the molarity of caffeine in coffee?

Mol.wt. caffeine = 8(12)+10(1)+4(14)+2(16) = 194
125 mg in 250 mL 4(125) mg in 1 L
500 mg = 0.500/194 mol = 2.58 x 10³ mol

Concentration is 2.58 x 10³ M

2. Pure acetic acid is a liquid with a density of 1.049 g/mL at 25C.
(a) Calculate the molarity of a solution of acetic acid made by dissolving 10.00 mL of the pure acid at 25C in enough water to make 100.0 mL of solution

molwt H(C₂H₃O₂) = 60.0
mass of 10.00 mL pure acid = (10.00 mL)(1.049 g/mL) = 10.49 g or 10.49/60 = 0.175 mol
0.175 mol in 100 mL 1.75 mol in 1 L

Answer: 1.75 M

(b) Will the molarity of this solution increase, decrease, or remain unchanged if the temperature is raised to 60C?

Molarity will decrease (volume of solution increases)

3. (a) List two components (there are many) that contribute to the Internal Energy of a system comprising 50 mL water contained in a beaker.

kinetic energies of the water molecules
attractions between the water molecules
vibrational energies of the water molecules
electron-nuclear attractions

(b) If the system absorbs 327 kJ heat from the surroundings and does 430 kJ of work on the surroundings, what is E?

E = q + w = +327 - 430 = -103 kJ

(c) Why is H a more useful quantity than q to work with in Chemistry?

Unlike q, H is a State Function and is independent of the number and type of steps that are involved in tranforming the system from its initial to its final state.

4. (a) Complete and balance the following equations in ionic form:

(a) Al(NO₃)₃ (aq) + ZnBr₂ (aq)
b) Al (s) + ZnBr₂ (aq)
(c) ZnCO₃ + H⁺
(d) Ba(OH)₂ + HNO₃
(e) Hg₂(NO₃)₂ (aq) + NH₄I (aq)
(f) H⁺ + ClO₄^-

(a) Al³⁺ + NO₃^- + Zn²⁺ + Br^- ---> NR

(b) 2 Al + 3 Zn²⁺ ---> 2 Al³⁺ + 3 Zn

(c) ZnCO₃ + 2H⁺ ---> Zn²⁺ + CO₂ + H₂O

(d) OH^- + H⁺ ---> H₂O

(e) Hg₂²⁺ + 2I^- ---> Hg₂I₂

(f) H⁺ + ClO₄^- ---> NR

(b) In the laboratory, how could you distinguish
(i) sodium chloride (table salt) from sodium iodide (a poison)?

To solution add AgNO₃. Chloride gives white AgCl ppt, iodide gives yellow AgI ppt.
Or heat with conc H₂SO₄: chloride gives colorless HCl (g); iodide gives violet iodine vapor.
Other methods also

(ii) solid barium chloride from solid barium sulfate ?

chloride is soluble in water, sulfate is insoluble.
Other methods also

5. The Chemistry Department has recently ordered a nuclear magnetic resonance (NMR) spectrometer that will operate with radiation of frequency 500 MHz.
(a) What is the wavelength of this radiation?

l = c/n  = (3.00 x 10⁸ m/s)/(500 x 10⁶ s^-1)

= 0.600 m

An einstein is a mole of photons.
(b) How many kJ are contained in an einstein of radiation from the NMR spectrometer?

The energy of one photon = hn  = 6.63 x 10^-34 J-s(500 x 10⁶ s^-1) = 3.32 x 10^-25 J

The energy of one einstein = 3.32 x 10^-25(6.02 x 10²³) = 0.200 J

(c) How many einsteins of this radiation would be needed to raise the temperature of 10.0 g water from 20.0 to 21.5C?

Joules needed to raise temp. of water = 10.0(21.5-20.0)(4.184) = 62.8

number einsteins needed = 62.8/0.200 = 314

6. (a) For any four of the following scientists write a sentence that indicates his contribution to our understanding of the electronic structure of atoms: Einstein, Heisenberg, de Broglie, Pauli, Bohr, Planck.

Einstein: gave an explanation for the Photoelectric Effect in terms of the energies of the photons that knock electrons off metal surfaces.

Heisenberg: proposed the Uncertainty Principle that the precise position and momentum (or velocity) of a particle cannot be determined simultaneously.

de Broglie: postulated that matter could exhibit properties of waves.

Pauli: the Exclusion Principle - no two electrons in an atom can have the same set of four quantum numbers.

Bohr: the first quantum mechanical model of the hydrogen atom that explained the characteristic line spectrum of H.

Planck: proposed the that energy is not continuous but is emitted or absorbed by atoms in very small packets (quanta)

(b) What are the possible values of quantum numbers for a 3d electron?
a 3d electron has n = 3 and l = 2

Possible values of m_l are +2, +1, 0, -1, -2
Possible values of m_s are +½ and -½

(c) Which ground state atom has the electron configuration [Ar]3d⁸4s² ?

₂₈Ni

(d) Write the electronic configuration for the ground state of arsenic. Does an As atom have any unpaired electrons?
As: [Ar]3d¹⁰4s²4p³ OR 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p³

three electrons in the 4p subshell (can hold 6) means that there are three unpaired electrons in an arsenic atom (Hund's rule).

(e) In a potassium atom which kind of electron, 4s or a 3d, will experience a greater effective nuclear charge? Why should this be so?

Potassium is in the s-block of the Periodic Table, and its last electron is in a 4s orbital (rather than 3d). This means that a 4s electron in potassium feels a greater nuclear charge than a 3d electron.

The reason for this is that the 4s orbital has some of its electron density close to the nucleus, "inside" the inner shells of electrons. The 3d orbital has most of its electron density outside the inner shells of electrons

7. A sample of CuO weighing 3.50 g was dissolved in 500.0 mL of a solution of HCl (A). The resulting solution (B) was still acidic and a 50.00-mL sample was titrated with 0.0850 M NaOH and required 19.50 mL of the alkali for complete neutralization.
Calculate (a) the molarity of B;

(a) In titration, 50.00(M_B) = 19.5(0.0850)
M_B = 0.0332 M

(b) the molarity of A
(b) # mol HCl left in 500 mL of A = (0.0332 mol/L)(0.500 L) = 0.0166

Each mol CuO requires 2 mol HCl for neutralization.
# mol CuO = 3.50/79.5 = 0.044
# mol HCl consumed by CuO = 2(0.044) = 0.088
Therefore the total number of moles of HCl in solution A

= 0.0880 + 0.0166 = 0.1046

Concentration of HCl in solution A = 0.1046 mol/0.5 L = 0.209 M

8. Write the following equations in ionic form:
(a) 2KMnO₄ + 8H₂SO₄ +10AgBr ---> 5Br₂ + 2MnSO₄ + 5Ag₂SO4 + K₂SO₄ + 8H₂O

(a) 2MnO₄ + 16H⁺ + 10 AgBr ---> 5Br₂ + 10 Ag⁺ + 8H₂O

(b) 8CuSCN + 22HNO₃ + 27H₂O ---> 8CuSO₄ + 8CO₂ + 15NH₄NO₃

(b) 8CuSCN + 15H⁺ + 7NO₃ + 27H₂O ---> 8Cu²⁺ + 8SO₄² + 8CO₂ + 15NH₄⁺

9. Which of the following reactions is more exothermic? Why?
(a) CH₃OH (g) + 3O₂ (g) ---> 2CO₂ (g) + 4H₂O (l)
(b) CH₃OH (l) + 3O₂ (g) ---> 2CO₂ (g) + 4H₂O (l)

Reaction (a) is more exothermic, because in (b) heat must be supplied to the system to convert CH₃OH (l) to CH₃OH (g).

10. Calculate the enthalpy change for the following reaction
Na₂CO₃ (s) + H₂O (g) + CO₂ (g) ---> 2NaHCO₃ (s)

DH = 2(-947.7) - {-1130.9 + (-241.8) + (-393.5)}
= -1895.4 + 1766.2 = -129.2 kJ