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chem 127
Exam 3
October 27, 2004

(The last page contains useful data )- not provided on Web version

I.. Short answers (Chemical Equations, Equilibrium Expressions)

1. (6 pts) The weak base methylamine CH₃-NH₂ reacts with water

write the chemical equation for this reaction
write the equilibrium expression for this reaction
K_b = ________________________

2. (6 pts) Write the equilibrium expression for the solubility of the salt Ag₂SO₄

write the chemical equation for the reaction involved
write the equilibrium expression for this reaction
K_sp = _____________________________

3. (6 pts) Oxalic Acid is a diprotic acid (write it as H₂Ox)

write the equilibrium expression associated with the second ionization step
K_a2 = ______________________________

4. (6 pts) The hexaaqua copper(II) ion reacts with ammonia, NH₃, and it forms the tetraamminediaqua copper(II) ion

write the chemical reaction
and write the equilibrium expression for that reaction

5. (6 pts) We begin with a solution that contains 0.10 mole of HNO₃ and 0.05 moles of Acetic Acid; we then add 0.075 moles of the base Ammonia (NH₃). What remains in solution?

H₃O⁺ ______ moles
NO₃ ______ moles
Acetate Ion ______ moles
Acetic Acid ______ moles
NH₃ ______ moles
NH₄+ Ion ______ moles

6.(10 pts) Some small molecules or ions (such as NH₃, H₂O, CN^- and Cl^-) act as ligands, binding to metal ions like Ni²⁺. Such ligands act as Lewis Bases and form Coordinate Covalent Bonds.

What is the definition of a Lewis base?
What is the essential feature of these ligands ?

7. (10 pts) (Refer to the data and figures on the last page, icluding periodic table.)

a. What is the electron configuration of the Cr³⁺ ion ?
b. Why is Cr(H₂O)₆3+ ion colored ?
c. How would the color of Cr(CN)₆3- ion differ in color ?
- (Remember, CN- is a stronger ligand)

II. Calculations

8. (20 pts) A 25.00 sample of 0.10 M Acetic Acid, HA is titrated with 0.10 M NaOH.

Ka= 1.8 x 10^-5. for acetic acid
The equivalence Point is reached at 25.00 ml of base.

Compute the pH during the titration at any two points from this list
- a. At 0.00 ml of NaOH,
- b. at 12.50 ml of NaOH
- c. at 24.90 ml of NaOH
- d. At 25.00 ml of NaOH
- e. at 30.00 ml of NaOH

9. (15 pts) The salt Cu(OH)₂ has Ksp = 1.6 x 10^-18.

What is the solubility (moles/liter) of Cu(OH)₂ ?
9a. Explain why it is less soluble in 3M NaOH but is more soluble in 3M NH₃ (a base that also acts as a source of OH- )

10. (15 pts) Acetic Acid, HA, has Ka = 1.8 x 10^-5

a. what is pKa for Acetic Acid ?
b. we say we can compute Kb for acetate ion as Kw / Ka
- show why that is true.

(As an alternative to part b, give directions for preparing a buffer that has pH = 4.00

using only 0.50 M Acetic Acid, 0.50 M Sodium Acetate and water.)
(end of exam)

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