Nazareth Sec0ndary School 2nd Term Exam In Chemistry Jan – 2013

Science Sections Duration: 90 min

Question (I): (4 points)

I- A solution (B) of sulfuric acid (H2SO4) was prepared by diluting 100 times solution (A) of PH =2.

1- Calculate the concentration of solution (B) then deduce its new PH.

2- Do you think the given solution is a strong acid? “Justify”

3- What is the effect of dilution on the concentration and PH of the given solution?

II- 5 g of HClO4 dissociates completely in water to prepare a solution of volume 2 L.

a- Determine the concentration of H3O+ in this solution then deduce the concentration of OH- . b- Calculate its PH and POH. Given: - • M (Cl) = 35.5 g\mol, M (H) = 1 g\mol and M (O) = 16 g\mol.

• The ion product of water Kw = 10-14

Question (2): (7 points)

Benzoic acid “C6H5COOH” is a white crystalline solid slightly soluble in water. It is used as a preservative in soft drinks.

Given: • This study is done at 25 ̊C; the solubility of benzoic acid in water is 2 g.L-1

• Molar mass of benzoic acid is 122 g.mol-1

A mass 305 mg of benzoic acid is introduced into 250 ml volumetric flask, half filled with distilled water. Then, the necessary steps are followed until a solution (S) is obtained.

1-List the materials needed to prepare solution (S) then describe the procedure of this preparation.

2-Verify that solution (S) is not saturated.

3-Calculate the concentration of solution (S).

4-40 ml of solution (S) is mixed with 20 ml of sodium hydroxide solution of concentration Cb = 1.0×10-2 mol.L-1.

a-Write the equation of the reaction that takes place

b-Determine the mole composition when the reaction is over then deduce the concentration of every ionic species in the solution.

Question (3): (3 points)

Consider the reaction:- ClF3 (g) + NH3 (g) → N2 (g) + HF (g) + Cl2 (g) …… ΔH = - 1196 Kj /mol.

1-Balance the given reaction

2-Is this reaction exothermic or endothermic? “Justify”

3-Calculate the heat of formation of ClF3 (g) where ΔH ̊f of HF (g) = -271 Kj /mol and ΔH ̊f of NH3 (g) = - 46 Kj/ mol

Question (4): (6 points)

The label on a bottle containing commercial hydrochloric acid solution (S0) shows the following information: M (HCl) = 36.5 g/mol, density = 1.2 Kg/L and % by mass = 37%

I-Dilution of solution (S0):

To prepare 1 L of hydrochloric acid solution (S), a volume V0 = 8.3 ml of (S0) is introduced into 1000 ml volumetric flask.

a-Describe the experimental procedure to be followed in this preparation.

b-Show that the molar concentration Cs of solution (S) is nearly 0.1 mol.L-1.

II-Titration of solution (S) using PH-meter:

In order to determine the concentration Cs, solution (S) is titrated with a volume Vb =20 ml of freshly prepared sodium hydroxide of concentration Cb = 4.0 × 10-2 mol.L-1.

The PH-reading during the course of titration are given in the table below:

V (S) ml |2 |4 |6 |8 |9 |10 |11 |12 |13 | |PH |12.4 |12.2 |11.9 |11.1 |2.7 |2.3 |2.1 |2.0 |1.8 | |1-Why do we use a recently prepared sodium hydroxide solution?

2-List all the materials used during titration

3-Plot, on a graph paper, PH = f (Vs) where 0 ≤ Vs ≤ 12 using the given scale: 1 cm = 2 ml and 1 cm = 1 PH unit.

4-Write the equation of titration and give its characteristics

5-Determine the concentration Cs of solution (S) then find the concentration of (S0).

6-At the end of titration, the content of the buret is let to run into the beaker. Compare the PH of the obtained solution to that of solution (S). “Justify”

GOOD LUCK