Hariri High School II

Chemistry Test

Name: ______Date: Feb. 1st, 2011

Grade: 12 (G.S & L.S)Time: 100 min.

Degree of Vinegar

-Applying Knowledge

-Mastering Communication Skills

-Mastering Experimental Techniques

CH3COOH is a weak acid of pKa = 4.75. Its molar mass is 60g/mol. The degree of vinegar is the mass of acid, expressed in grams, contained in 100g of vinegar.

1. Write the equation of the reaction between CH3COOH and NaOH. (0.5pts.)

2.Write the expression of KR of the reaction and calculate its value. Deducewhether the reaction is quantitative or not. (1pt.)

B. Two different commercial samples of vinegar A and vinegar B are diluted10times to prepare 100mL of each solution SA and SB. To describe the procedure followed, choosethe suitable items from the available equipment: (1.5 pts.)

50mL and 100mL beakers.
10mL, 25mL and 50mL graduated cylinders.
50mL graduated buret.
5mL, 10mL and 20mL graduated pipets.
50mL and 100mL volumetric flasks.
pH electrode.
Magnetic stirrer.

C. 10mL of the diluted vinegar is to be titrated to find its concentration. Thus,enough water is added to immerse the electrode, then 0.1mol/L NaOH isadded gradually for titration.

1. Choose from the above list the equipment needed for titration. (0.5pts.)

2. Does the addition of distilled water affect the results of the titration? Why? (0.5pts.)

D. The pH values as a function of the volume of NaOH added are plotted. Twocurves A and B are obtained due to the titration of solutions SA and SB.

1. Determine the equivalence point from each curve. (2.5pts.)

2. Calculate the concentrations of SA and SB. Deduce the concentrations of thecommercial solutions A and B. (3pts.)

3. The commercial solution has a density of 1Kg/L; calculate the degree of eachvinegar, knowing that the tested vinegar solutions are labelled 5o and 8o. Assigneach curve to its corresponding vinegar. (2pts.)

Identification of Solutions

-Applying Knowledge

-Mastering Communication Skills

Seven beakers contain different solutions of the same concentration C = 1x10-2mol/L and the same volume V = 100mL. The beakers are numbered, and the pH of each solution is measured at 25oC.

Beaker No. / 1 / 2 / 3 / 4 / 5 / 6 / 7
pH / 2.9 / 7 / 5.6 / 11.3 / 10.6 / 3.1 / 12

Each solution is prepared by dissolving in distilled water one of the following seven compounds: Sodium chloride- sodium hydroxide- ammonium chloride-ammonia solution- benzoic acid(C6H5COOH)- methanoic acid(HCOOH)-methyl amine (CH3NH2).

Given: pKa(NH4+/NH3) = 9.2

pKa(CH3NH3+/CH3NH2) = 10.6

pKa(C6H5COOH/C6H5COO-) = 4.2

pKa(HCOOH/HCOO-) = 3.7

Arrange the above couples on the pKa axisand then identify the solutions ineach beaker. Justify your answer. (2.5pts.)

Lactic acid of formula CH3CH(OH)COOH is formed by the fermentation oflactose found in milk. This acid is represented by HA. Its pKa is 3.9 at 37oC.

A. The pH of fresh milk is 6.4 at 37oC. Find the ratio [A-]/[HA]. Deduce thepredominant species in milk. (2pts.)

B. One of the couples that regulates the pH of blood is H2PO4-/HPO42- ofpKa = 7.2. The pH of blood is 7.4.

1. Calculate the ratio [HPO42-]/[H2PO4-] in blood. (1pt.)

2. The concentration of HPO42- is 0.275mol/L in the above-mentioned blood sample.

Deduce [H2PO4-]. (0.5pts.)

3. A reaction takes place in blood, producing 0.05mol of lactic acid per oneliter of blood. Write the equation of the reaction taking place between HAand HPO42-; then calculate its constant. (1pt.)

4. Suppose that the reaction is total; find the concentration of HPO42- and H2PO4-. Verify that the pH of blood becomes 7.2. (1.5pts.)