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Chem 18a Test 2 Example problems - solutions

1) Multiple choice

a) The wavelength of red light is 680 nm.  What is the wavenumber of red light in cm- 1?       (A)  6.80× 10-6             (B)  1.47 × 106                   (C)  1.47 × 105              (D)  1.47 × 104      (E)   none of these

b) Which  color of visible light from the list below has the longest wavelength?

A) violet      B) blue     C) yellow     D) green

c) If larger than prescribed amount of sulfuric acid is added to your product after weighing it for titration, how it will affect the determination of wt % of oxalate in your product?

A: lower wt% of oxalate than actual       B: higher wt% of oxalate than actual           C: no change

d) The oxidation state of manganese in potassium permanganate is:

A) +5     B) +7    C) +2     D)+4      E) None of the above

2) List all safety hazards in this picture:

1. She should tie back her long hair.

2. She should wear a shirt with tighter fitting sleeves.

3. She should put on safety goggles.

4. She should remove the can of acetone near the burner.

3) List all required properties of a primary standard and explain what is primary standard used for.

Primary standard must be of high purity, low moisture absorption, low cost, stable in air, soluble, and have a large molar mass.

Primary standard is a pure substance used in the standardization process of a titration (to determine molarity of a titrant).

4) Write the balanced chemical equations for the reactions experimentally studied in the general chemistry lab to determine the heat of formation of MgO by calorimetry.  In the equations enter the states of reactants and     products (aq – aqueous, g – gas , s – solid, l – liquid).

Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)

MgO(s) + 2HCl(aq) → H2O(l) + MgCl2(aq)

6) How many moles  of KMnO4 are needed for the titration of  100. mL  of 0.30 M  K3 [Al(C2O4)3]? # mol KMnO4 = # moles K3 [Al(C2O4)3 × ×

7) (10) The weight % of oxalate in sample of 0.315 g of K2 [Cu(C2O4)2]•2H2O is measured by  titration with  0.0199 M solution of KMnO4 . The equivalence point is reached when 33.84 mL of KMnO4 solution is added. What is the wt% of oxalate in the sample?

# moles of KMnO4 = M * V (L) = 0.0199 mol/L * 0.03384 L = 6.734×10-4 moles KMnO4 (3)

# mol C2O4 = mol KMnO4 × = 1.684×10-3 moles of C2O4 (4)

moles of KMnO4*MMC2O4 1.684×10-3*88.04 g/mol

wt. of sample 0.315 g

8) The absorptivity of Fe(o-phen)32+ at 510 nm = 1. 10x104  cm- 1M- 1 . If a solution exhibits an absorbance of 0.20 at 510 nm when measured in a 1.00 cm cell, how many mg of Fe are present in  1 L of solution?

A = abc => c = A/ab = 0.20/(1.10× 104 cm-1M-1 × 1 cm) = 1.8× 10-5 M

AWFe = 55.85 g/mol

55.85 g 1000 mg

9) Define:      a) wavenumber: reciprocal value of wavelength

b) infrared spectrum: plot of transmittance T ( or %T) of infrared radiation vs. wavenumber for an analyzed sample.

c) heat of formation: quantity of heat produced when 1 mole of a compound is formed from its elements

d) standardization: determination of molarity of a solution by titration