CHEM100 Exercises for final exam

1. Algorithmic questions (3 marks each), 3x4=12 marks

A) Calculate the pH of an NaOH solution prepared by adding 200.0 mL of water to 100.0 mL of a 0.010 M solution of the base.

B) Calculate the pH of the solution obtained by adding 20mL of 0.01M solution of Sr(OH)₂ to 40 mL of 0.015M solution of HCl

C) Calculate the reaction enthalpy, ∆H⁰ from the ∆H⁰_f for the following reaction:

2 CO(g) + O₂ (g) à 2CO₂ (g)

(∆H⁰_f is -100.5 and -393.7 kJ/mol for CO and CO₂, respectively)

D) A compound composed of C, H, and O contains 70.6%C, 5.9%H, and 23.5% O by mass. The molecular weight of this compound is 136amu. What is the molecular formula?

2. Theoretical part (3 marks each) 3x8=24

A) What is Gibbs energy and how it is used to predict spontaneity of chemical reactions?

B) What is ionization energy and electron affinity of an atom? Write down corresponding reactions.

C) What is covalent and ionic radius?

D) Define 3 laws of thermodynamics

E) For certain reaction, you know the activation energy and the Gibbs free energy. Which one will you use to calculate the rate constant for this reaction?

F) Define” core” and “valence” electron shells. Estimate Z_ef for carbon atom. How Z_ef changes across the period in the Periodic table?

G) Which sign (plus or minus) would you use for a work which is done on the system by the surrounding?

H) A buffer (1L) contains 0.1 mol of acetic acid and 0.3mol of sodium acetate. Does the buffer have greater capacity to bind a strong acid or a strong base? Explain why a buffer could hold its pH after a small amount of a strong acid or a strong base is added.

3. Long Answer questions

A) {12} Consider the following elements and ions:

O, F^- Na, Fe³⁺, N^3-, Sc , Sr, Br

a) Write down condensed electron configurations for metals (elements and ions) listed above

b) Find out the isoelectronic series and list it in order of increasing radius

c) Arrange the elements nonmetals listed above in order of increasing fist ionization energies

d) Write down formula for an ionic compound composed from the largest nonmetal and smallest metal listed above

B. {12} Draw the Lewis structure(s) for N₂O⁺ ion. Report number of valence electrons. Show non-bonding electron pairs, resonance structures, and formal charges. Is the ion linear or bent? What is hybridization on central atom?

C. {8} A chemical compound, B, decomposes at 298K, with the rate constant k=1.2 x 10^-3 s^-1.

a) Write down the rate law for the decomposition reaction

b) Calculate the half-life for this reaction

c) Calculate the percentage of B , which is left after 20 min.

d) Calculate the rate constant at 398K if the activation energy for decomposition is 50 kJ/mol

D. {6} The following data were obtained for the reaction A+BàP

Experimental number [A], M [B], M initial rate M/s

1 0.273 0.763 2.83

2 0.273 1.526 2.83

3 0.819 0.763 25.47

a) deduce the rate law for the reaction

b) calculate the rate constant

c) find the initial rate for [A]=0.1M and [B]=0.2M

E. {5} Consider the following equilibrium: 2HI(g) D H₂ (g) + I₂ (g). At 425 ⁰C, K_eq= 1.84 x10^-2. Calculate equilibrium pressure of I₂ , if P_eq =0.096 atm for H₂ and 0.708 atm for HI

F. {5} Determine the K_sp for Mg(OH)₂ if the solubility of Mg(OH)₂ is 1.4 x 10^-4 M

G. {10} How many grams of sodium acetate (CH₃COONa) should be added to 1.00 L of 0.818M solution of acetic acid (CH₃ COOH) in order to have a buffer with pH=4.1? Calculate the pH of the buffer after addition of 0.001 mol of NaOH. (assume volume of 1.00 L for all calculations)

H. {6} Consider the following reaction:

2PCl₃ (g) à P₂ (g) + 3Cl₂ (g)

Substance ∆H⁰_f (kJ/mol) S (J /mol K)

PCl₃ (g) -288.1 311.7

P₂ 144.3 218.1

Cl₂ 0.0 222.96

Using the data from the table, calculate K_eq for this reaction.

At which temperature this reaction will be spontaneous?