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CBSE Class 12 Pre Board 2021 : Chemistry (Arvind Gupta D A V Centenary Public School, ) : Pre board

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Sutapa Chakraborty
Arvind Gupta D A V Centenary Public School
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D.A.V. PUBLIC /MODEL SCHOOLS, WEST BENGAL ZONE PRE- BOARD EXAMINATION Session: 2020-2021 Class: XII Maximum Marks :70 Subject: Chemistry Time: 3 Hours General Instructions : a) There are 33 questions in this question paper of 6 pages. All questions are compulsory. b) Section A : Q. No. 1 to 16 are objective type questions . Q. No. 1 and 2 are case based questions having four MCQs or Assertion Reason type based on given passage each carrying 1 mark. Q. No. 3 to 16 are MCQs and Assertion Reason type questions carrying 1 mark each. c) Section B : Q. No. 17 to 25 are short answer Type I questions and carry 2 marks each. d) Section C: Q. No. 26 to 30 are short answer Type II questions and carry 3 marks each. e) Section D : Q. No. 31 to 33 are long answer questions and carry 5 marks each. f) There is no overall choice . However internal choices have been provided . g) Use of calculators and log tables is not permitted. _____________________________________________________________________________________________ 1 SECTION A (OBJECTIVE TYPE ) Read the passage given below and answer the following questions: (1x4=4) Alcohols and phenols react with active metals such as Na , K and Al to yield corresponding alkoxides / phenoxides and hydrogen. The acidic character of alcohols is due to the polar nature of O-H bond. Alcohols are however weaker acids than water. Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors. The hydroxyl group in phenol is directly attached to the sp2 hybridized carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule causes the oxygen of -OH group to be positive. Presence of substituents affects the acidic strength of alcohols and of phenols. The following questions are multiple choice questions. Choose the most appropriate answer: (i)The correct decreasing order of acidity of the following compounds is p-Nitrophenol(I), o- Nitrophenol(II), Phenol(III), m- Nitrophenol(IV) a) I>II>III>IV c) I>II>IV>III b) II>I>IV>III d) II>I>III>IV (ii) Which one of the following is the strongest conjugate base? a) (CI3)3COb) (CH3)3COc) (CCl3)3CO d) (CBr3)3COOR Which one of the following is the correct decreasing order of acidity? a) Water > Alcohol > Phenol > Acetic acid b) Acetic acid > Phenol > o-nitrophenol > alcohol c) Chloroacetic acid > phenol > water > alcohol > 1-alkyne d) p-nitrophenol > chloroacetic acid > water > 1-alkyne

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