NJC_Prelim_P1_Detailed_solutions

1 NJC Preliminary Examination 9647/01/15 [Turn Over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 1 Multiple Choice Additional Materials: Multiple Choice Answer Sheet Data Booklet 9647/01 Thu 17 Sep 2015 1 hour READ THESE INSTRUCTIONS FIRST Write in soft pencil. Write your name, subject class and registration number on the Optical Answer Sheet (OAS) in the spaces provided unless this has been done for you. There are 40 questions in this paper. Answer all questions. For each question there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the separate Optical Answer Sheet. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. This booklet consists of 19 printed pages and 1 blank page.

2 NJC Preliminary Examination 9647/01/15 [Turn Over Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1 When scientists research on atomic nuclei, they often correlate the stability of isotopes with the proton to neutron ratio. Which isotope has the same proton to neutron ratio as 18 8 O? A 9 5 B B 26 12 Mg C 36 16 S D 52 24 Cr Given: n p ratio of 18 8 O = 8 - 18 8 = 5 4 n p ratio of 9 5 B = 4 5  5 4 ; n p ratio of 26 12 Mg = 14 12 = 7 6  5 4 ; n p ratio of 52 24 Cr = 28 24 = 7 6  5 4 n p ratio of 36 16 S = 20 16 = 5 4  ans is C 2 AxOy, a gaseous oxide of an element A, gives an acidic gas AO2 on combustion with excess oxygen. When 10 cm 3 of AxOy is burnt in 40 cm 3 oxygen (in excess), the total volume of gas after cooling to the original temperature is unchanged . On shaking with excess aqueous potassium hydroxide, the volume is reduced to 20 cm 3. All volumes are measured at the same temperature and pressure. What are the values of x and y? x y A 1 1 B 2 3 C 3 2 D 3 4 After burning, total volume = 50 cm3 = vol of unreacted O2 + vol of AO2 Since O2 does not react with KOH(aq), AO2 is the acidic gas that reacts with KOH(aq) Vol. of unreacted O2 = 20cm3 Vol. of AO2 = 30cm3 Eqn for reaction: AxOy + (x – ½y)O2  xAO2 Initial vol / cm3 10 40 0 Final Vol after burning / cm3 0 20 30 Vol reacted/produced/ cm3 10 20 30 Reacting mol ratio 1 2 3 x=3 x½ y = 2, ½ y = 1, y = 2

3 NJC Preliminary Examination 9647/01/15 [Turn Over 3 The diagram below shows the first ionisation energy of some element s with atomic numbers less than 20. Whi