Monday, December 17, 2007

Past JEE Questions Ch.1

JEE Question 2007 paper I Linked Comprehension

Chemical reactions involve interaction of atoms and molecules. A large number of atoms/molecules (approximately 6.023 ×10^23 ) are present in a few grams of any chemical compound varying with their atomic/molecular masses. To handle such large numbers conveniently, the mole concept was introduced. This concept has implications in diverse areas such as analytical chemistry, biochemistry electrochemistry and radiochemistry. The following example illustrates a typical case, involving
chemical/electrochemical reaction, which requires a clear understanding of the mole concept.

A 4.0 molar aqueous solution NaCl is prepared and 500 mL of this solution is electrolysed. This leads to the evolution of chlorine gas at one of the electrodes (atomic mass : Na = 23, Hg = 200;1 Faraday = 96500 coulombs).

1. The total number of moles of chlorine gas evolved is
(A) 0.5
(B) 1.0
(C) 2.0
(D) 3.0

Answer: B

2. If the cathode is a Hg electrode, the maximum weight (g) of amalgam formed from this solution is

(A) 200
(B) 225
(C) 400
(D) 446

Answer: D

In presence of Hg cathode sodium ion will discharge in place of hydrogen gas due to over voltage in the form of amalgams.

Weight of amalgam = 2 × (23 + 200) = 446 g

3. The total charge (coulombs) required for compete electrolysis is

(A) 24125
(B) 48250
(C) 96500
(D) 193000

Answer: D

Total charge = 2 × 96500 = 193000 C

This question is better answered after studying the electro chemistry chapter.

JEE Question 2007 paper II

Consider a titration of potassium dichromate solution with acidified Mohr’s salt solution using diphenylamine as indicator. The number of moles of Mohr’s salt required per mole of dichromate is

(A) 3
(B) 4
(C) 5
(D) 6

Answer D

From Compounds of metals chapter
Mohr's salt is a double salt. FeSO-4.(NH-4)-2SO-4.6H-2O
It is obtained by mixing freshly prepared ferrous sulphate in solution with equal molar amounts of ammonium sulphate and then allow the solution to crystallize.

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