SCHOLASTIC APTITUDE TEST 1999

CHEMISTRY

Instructions:

(1) Answer must be written either in English or the medium of instruction of the candidate in high school.

(2) There will be no negative marking

(3) Use of calculators or graph papers is not permitted

(4) There are SIX Questions. Answer all the questions.

I. Multiple choice. Each question has only one correct answer. Indicate the correct answer by A,B,C,D. (5x1 = 5 Marks)

1. The electronic configuration of elements A,B,C and D are (2,8,1) (2,8,2) (2,8,6) and (2,8,7) respectively which of them can make an ion with two negative charges.

2. A neutral atom of an element has a nucleus with a nuclear charge 13 times and mass 27 times that of hydrogen nucleus. How many electrons would be in its stable positively charged ion.

3. At any moment before reversible reaction attains equilibrium it is found that

A) The velocity of the forward reaction is increasing and backward reaction is decreasing

B) The velocity of backward increasing and forward decreasing

C) The velocities of both forward and backward reaction are increasing

D) Velocities of both forward and backward reaction decreasing

4. When the same amount of Zn is treated separately with excess of sulphuric acid and excess of sodium hydroxide the ratio of volumes of hydrogen evolved is

5. Which of the four conditions density of the nitrogen gas will be largest

II. Write the balanced Chemical Equations for following chemical changes:                               (5x1=5 Marks)

1. Zn granules are dropped in to very dilute solution of nitric acid.
2. Mixture containing CO₂ and CO is passed over the redhot coke.
3. Ammonium nitrate is strongly heated.
4. White phosphorus is treated with caustic soda solution.
5. KI is treated with con.H₂SO₄ solution.

III. Explain the following Facts: (10x2 = 20 Marks)

(Write the chemical equations wherever necessary)

1. NO₂ is reddish brown gas but on cooling it becomes colourless but on heating again it gains the colour.
2. Potassium chromate in water forms yellow solution. On addition of acid this solution turns orange, on addition of NaOH to the orange solution again becomes yellow.
3. Calcination is carried in the absence of air whereas roasting is carried in the presence of air.
4. Gold is insoluble in either in con. HCl (or) HNO₃ separately while it is soluble in aqua-regia a mixture of both.
5. I^st Ionisation energy of Nitrogen is greater than oxygen where as reverse the case for 2^nd Ionisation value.
6. To protect the Iron from corrosion it is covered with zinc (Galvanisation) even though zinc is more reactive than Iron.
7. When Nitric acid is exposed to light slowly turns yellow.
8. In the detection of through Brown ring test, concentration H₂SO₄ slowly added through the walls of Test tube.
9. Ammonium chloride is a salt, but its aq.solution terns blue litmus red.
10. Metallic zinc displaces Hydrogen gas from dilute H₂SO₄ but metallic copper doesnot.

IV. Fill in the blanks (5x1 = 5 Marks)

1. Ratio of weights of O₂ and O₃ is 1:1 so the ratio of their moles is ___________________.
2. 250 ml of 6M HCl, 650 ml of 3M HCl were mixed together. The volume of water should be added to make the solution 3M is _____________________ ml.
3. If a hospital buys 8 grms of Co⁶⁰, the amount of same cobalt left after 20 years would be (t_1/2 of Co⁶⁰ = 5 years) _________________.
4. The central atom of a molecule of the type AX₃ has one lone pair in its valance shell. The shape of the molecule is _______________.
5. An element which can exist as (+) ve ion in acidic solution but also as a negative ion in basic solution is said to be ______________________.

V. Differentiate the following: (10x2 =20 Marks)

1. oils - fats
2. ionisation - electrolysis
3. thermoplastics - thermosetting plastics
4. soap - detergent
5. sublimation - liquification
6. metallic conductor - electrolytic conductor
7. hygroscopic solids - deliquescent solids
8. cathode - anode
9. bond energy - binding energy
10. ionic product - solubility product

VI. Solve the following: (2x2½=5 Marks)

1. Sodium bicarbonate (baking soda), NaHCO₃, can be purified by dissolving it in hot water (60⁰C), filtering to remove insoluble impurities, cooling to 0⁰C to precipitate solid NaHCO₃, and then filtering to remove the solid, leaving soluble impurities in solution. Any NaHCO₃ that remains in solution is not recovered. The solubility of NaHCO₃ in hot water at 60⁰C is 164 g/L. Its solubility in cold water at 0⁰C is 69 g/L. What is the percent yield of NaHCO₃ when it is purified by this method?
2. Hash Chemical Company sells a test kit for determining the concentration of chloride ion in domestic water supplies. The kit contains a silver nitrate, AgNO₃, solution that is added drop by drop to a 23.0 mL water sample to which an indicator has been added. When sufficient silver nitrate has been added to convert the chloride ion completely to silver chloride, AgCl, the solid produced turns orange. The concentration of the silver nitrate solution is such that each drop used to reach the color change corresponds to 12.5 mg of Cl^- per litre of water tested.

(a) What mass of chloride ion (mg/L) is contained in a water sample that requires 12 drops of test solution to reach the color change?

(b) What is the molar concentration of Cl^- in the sample tested in part(a)?

(c) If the sample size used is 5.75 mL instead of 23.0 mL, to what chloride ion concentration (mg/L) does 1 drop of the test solution correspond?

(d) If 20 drops of the silver nitrate test solution equals 1.00 mL, what is its molar concentration of AgNO₃?