2021_NJC_Prelim_P2_Ans

NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 1 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 8873/02 Tuesday 24 August 2021 2 hours READ THESE INSTRUCTIONS FIRST Write your name, index number, form class, tutorial class and subject tutor’s name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. The use of an approved scientific calculator is expected, where appropriate. Section A Answer all the questions. Section B Answer one question. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner's Use Question no. Marks Section A B7 B8 TOTAL /80 This document consists of xx printed pages and xx blank pages.

Section A Answer all the questions in this section in the spaces provided. 1 (a) Sulfur reacts with an element A to form a compound with empirical formula S A2. The percentage by mass of A in SA2 is 83.2%. (i) Assuming the relative atomic mass of A to be y. Find the number of moles of A and S respectively in terms of y. Element A S No. of moles/ mol 83.2 y 10083.2 32.1 or 0.523 [1] (ii) Hence, find the value of y. Mole ratio of A : S = 2 : 1 = 83.2 𝑦 : 100 −83.2 32.1 No. of moles of A atoms = 2  no. of moles of S atoms 83.2 𝑦 = 2  100−83.2 32.1 y = 79.5 [1] (iii) Element A is from Group 17. Hence, identify Element A using your answer in a(ii). Bromine [1] Sulfur has 21 unstable isotopes. Among these isotopes, the one with the longe st half-life of 85 days is 35S isotope. (b) (i) Suggest the time taken for 35S to decrease to 6.25% of its initial amount. 100%  50%  25%  12.5%  6.25% Time taken = 85 x 4 = 340 days [2]

(ii) When an atom of 40Ar collides with a high-speed neutron, 35S and another product is produced. State the identity of this product, including its mass and proton number. Ar_ 18 40 + nA_ 0 1 S + _____16 35 𝑯𝒆𝟐 𝟔 (conservation of protons and mass number) Left side Right side Mass number 40 + 1 = 41 35 + x = 41 x = 6 No. of protons 18 + 0 =18 16 + y = 18 y = 2 Hence the product’s mass number is 6 and number of protons is 2. From the Periodic Table, the element that has 2 protons is He. [1] (c) Disulfur dichloride, S 2Cl2, is an important precursor of the extremely toxic chemical warfare agent sulfur mustard. It was widely used during the First World War and the Iran–Iraq conflict. S2Cl2 can be synthesized at 200