Download Hariri High School II

Horj High School
Final Chemistry Test
Name:
Class: Grade 12 LS
Date: May , 2012
Time: 120min.
First Exercise (7pts.)
Buffer Solutions and Weak Bases
Parts I and II are independent
I- 0.25 mol/L of sodium hydroxide solution is added slowly from a buret into a beaker
containing 50 mL of 0.1 mol/L ethanoic acid (pKa = 4.75)
A- What volume of the base should be added to get a buffer solution?
B- Titration is completed and the following approximate graph is obtained. Find by
calculation,
1- the acidity constant Ka of ethanoic acid.
2- the initial pH of ethanoic acid solution before titration.
3- the volume of the base used at the point of equivalence.
4-Deduce the coordinates of points A and B(half equivalence). Justify
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II-Cyanic Acid Solution.
A cyanic acid (HCN) solution (S) of concentration 6x10-2 mol/L is prepared.
A- 10mLof solution (S) is added to 10 mL of distilled water. Find the pH of the
resulting solution, knowing that pKa (HCN/CN-) = 9.21. Then, find the value
of α, the degree of dissociation of HCN(aq).
B- 10mL of solution (S) is added to 10 mL of 2x10-1mol/L of NaCN solution.
1- Find the pH of the obtained solution.
2- Find the value of α', the degree of dissociation of HCN(aq).
3- Compare the values of α and α'. Justify your answer.
Second Exercise : (7pts.)
Identification Of Organic Compounds
The percentage by mass of oxygen in an organic compound (E) of general formula
CnH2n O2 is 24.61 %. (H=1 , C=12 , O=16)
A. Show that the molecular formula of (E) is C7H14O2.
B. The compound (E) may be an acid or an ester . What is the nature of ( E ),knowing
that its aqueous solution has a PH = 7 . Explain.
C. (E) can be prepared by the reaction of propanoic acid and a
saturated monoalcohol (A) of molar 74g. mol -1. Write,
1. the condensed structural formula of the propanoic acid.
2. the molecular formula of the alcohol (A).
3. the condensed structural formula and the name of each
of the isomers of (A) .
4. The mild oxidation of (A) by an acidified potassium permanganate solution
gives an organic compound (B). (B) reacts with the 2,4
dinitrophenylhydrazine ( D.N.P.H ) to give yellow crystals, but ( B )
has no reaction on a Fehling solution .
(a)-What is the nature of compound (B)? Write its condensed
structural formula and give its name.
(b)-Identify the alcohol (A) and write its class.
D. Write the equation of the reaction of the propanoic acid and the
alcohol (A) by using the condensed structural formulas. Give the name
of the organic compound (E).
E. 1mol of propanoic acid and 1mol of alcohol (A) are mixed at a certain
temperature T, in the presence of some mL of a concentrated sulfuric
acid. After a certain time an equilibrium is established, and the
efficiency (%  ) of the above reaction with respect to the formation of the
ester is 60%. Calculate the equilibrium constant Kc of the reaction.
F. Calculate the new value of the efficiency (%  '), if we mix initially one mole
of the propanoic acid and 5 moles of the alcohol (A) .Conclude
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Third Exercise (6pts.)
Alpha Amino Acids
Consider an α-amino acid (A) of general fornula:
R
CH COOH
NH2
One mole of α- amino acid (A) undergoes decaboxylation(losing CO2) reaction to
give one mole of CO2 and on mole of an amine (B).
A- Write the equation of the decarboxylation of (A). What is the class of the
amine (B).
B- We dissolve 0.081g of the amine (B) in a certain quantity of water in order to
form the solution (S). Write the equation of the reaction of the amine (B) with
water. What is the nature of the solution (S).
C- We titrate the solution (S) by a hydrochloric acid solution of concentration C
= 0.15 mol/L. The equivalent point is obtained by the addition of 12 mLof the
hydrochloric acid solution.
1- Calculate the number of moles of the amine (B) that reacted with the
acidic solution and deduce its molar mass.
2- Show that the molecular formula of (B) is C2H7N, write its condensed
structural formula and its corresponding name.
D- Write the structural formula and the name of the α- amino acid (A).
E- Show that the molecule (A) is chiral and represent its two enantiomers in
Cram representation.
F- The condensation of one molecule of (A) of name alanine and one molecule of
another α- amino acid glycine H2N-CH2-COOH produces a molecule of a
dipeptide and a molecule of water. Two reactions are possible. Write the
equations of these two possible reactions by using the condensed structural
formulas. Give the name each formed dipeptide.
Molar mass in g/mol of H,C and N are respectively : 1, 12 and 14
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