6
STRUCTURE QUESTIONS 1. The following table shows the relative atomic mass of a few types of carbon. Substance Relative atomic mass Carbon-12 12.000 Carbon-13 13.0003 Graphite 12.011 Diamond 12.011 iv. How do the structures of diamond and graphite explain their electrical conductivities respectively? v. The reaction C(diamond) C(graphite) ; ∆H = -3 kJ / mol is exothermic. Explain why this reaction does not occur spontaneously. [8] (b) Carbon-14 can be formed by the reaction between cosmic rays and nitrogen in the atmosphere. i. What are cosmic particles? ii. Write a balanced equation for the above reaction. [2] Comment by examiner : i. Since carbon-12 has high abundance compare to other, candidate should also include that carbon-12 has high relative abundance that other isotope [1] iv. Candidate only mentioned that diamond has strong covalent bond but did not stress that the strong bond build a giant molecular structure [1]. v. Candidate did not answer here. The reason even though ∆H is exothermic but it doesn’t form graphite easily was due to the high activation energy between diamond and graphite [1]. (b) i. cosmic particles are sub-particles that contain charge in it. ii. 14 7N + 1 0 n 14 6C + 1 1 p (a) i. Explain why carbon-12 replaced oxygen as a standard in the determination of relative atomic mass. ii. Why does the relative atomic mass of diamond differ from that of carbon-12? iii. Calculate the percentage of 12 C in graphite. PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry This document is not under copyrights of LRT Documents. The questions above is not printed question. 1 of 6

C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

Embed Size (px)

Citation preview

Page 1: C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

STRUCTURE QUESTIONS

1. The following table shows the relative atomic mass of a few types of carbon.

Substance Relative atomic

mass Carbon-12 12.000

Carbon-13 13.0003

Graphite 12.011

Diamond 12.011

iv. How do the structures of diamond and graphite explain their electrical conductivities

respectively?

v. The reaction C(diamond) � C(graphite) ; ∆H = -3 kJ / mol is exothermic. Explain why this reaction

does not occur spontaneously. [8]

(b) Carbon-14 can be formed by the reaction between cosmic rays and nitrogen in the atmosphere.

i. What are cosmic particles?

ii. Write a balanced equation for the above reaction. [2]

Comment by examiner : i. Since carbon-12 has high abundance compare to other, candidate should also include that carbon-12 has high relative abundance that other isotope [1]

iv. Candidate only mentioned that diamond has strong covalent bond but did not stress that the strong bond build a giant molecular structure [1]. v. Candidate did not answer here. The reason even though ∆H is exothermic but it doesn’t form graphite easily was due to the high activation energy between diamond and graphite [1].

(b) i. cosmic particles are sub-particles that contain charge in it.

ii. 147N + 1

0 n � 146C + 1

1 p

(a) i. Explain why carbon-12 replaced oxygen as a

standard in the determination of relative

atomic mass.

ii. Why does the relative atomic mass of diamond

differ from that of carbon-12?

iii. Calculate the percentage of 12C in graphite.

PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry

This document is not under copyrights of LRT Documents. The questions above is not printed question.1 of 6

Page 2: C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

2. Beams of particles travelling at the same speed from different sources are subjected to an electric

field as shown in the diagram below. A beam of neutrons has already been drawn

(a) Sketch and label on the diagram above how beams of each of the following particles are

affected by the electric field:

(i) protons (ii) electrons (iii) 2H+ [3]

(b) Explain briefly the position and shape of each beam.

(i) protons (ii) electrons (iii) 2H+ [3]

Comment by examiner : 2.a. since proton and 2H+ were positively charged so it will be deflected at negative terminal of electrical field, while electron was negatively charged ion, so it will be deflected at positively terminal 2.b)iii. Since 2H+ is 2 times heavier than a proton, so it was deflected 2 times further than proton. The angle of deflection by 2H+ is half of the proton.

3. (a) Describe the relative charges and masses of the three types of particles containing within the atom.

How do beams of these particles behave in an electric field? [6]

Comment by examiner : 3.a) The charge and mass was as stated by candidate, so 3 marks were given accordingly Candidate was able to state the deflection of particles in electrical field correctly, where neutron was no deflected, while proton was deflected to negative terminal and electron was deflected to positive terminal.

PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry

This document is not under copyrights of LRT Documents. The questions above is not printed question.2 of 6

Page 3: C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

(b) Because of differing proportions of isotopes, the relative mass of lead depends on its source. Two

samples of lead were subjected to analysis. The mass spectrum of the first sample was taken and the

following results were obtained

m/e value 204 206 207 208

relative abundance 27 48.0 41.5 100.0

When 1.00 g of the second sample was converted into lead (II) chloride, the mass of the product was

1.341g. Calculate the average relative atomic mass of each sample of lead, and decide whether or

not the two samples have identical isotopic compositions. [Take the relative atomic mass of chlorine

to be 35.45] [4]

Comment by examiner : 3.b) Candidate is able to calculate the RAM of Pb for the 1st sample using the average method, where the RAM of 1st lead sample is 206.87. Using the stoichiometri methods, the mole of chlorine gas was obtained, where mole of chlorine was 4.81 x 10-3 mol. From equation Pb2nd sample (s) + Cl2 � PbCl2 The candidate hence compare the mole of Pb to Cl2 and obtained the same number of mole, from the mole of Pb, RAM of Pb for the 2nd sample is obtained, which is 207.9 From both Pb, candidate is able to conclude that the sources of Pb is different.

4. The mass spectrum of naturally occurring magnesium is given below

24 25 26 m/e

(a) Name the ions that are responsible for the peaks

at positions 24, 25, dan 26 respectively.

(b) Calculate the relative abundance of each

isotope.

(c) Calculate the relative atomic mass of

magnesium. [6]

8

1 1

PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry

This document is not under copyrights of LRT Documents. The questions above is not printed question.3 of 6

Page 4: C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

Comment by examiner : 4.a) The 3 peaks given by magnesium at a m/e value close to each other, indicate Mg has 3 different isotope at different abundance 4.b) The relative abundance was then calculated by summing the total abundance and multiply with 100%, so 24Mg is 80% ; 25Mg and 26Mg are 10% 4 c) Candidate is able to calculate the relative atomic mass correctly using the information from the mass spectra given

5. Compound, X, containing only carbon, hydrogen, and oxygen, has the empirical formula C3H6O.

The mass spectrum of X is given below.

14 15 26 27 42 43 58 m/e

Comment by examiner : 5.a) Candidate only state the molecular formula but did not explain why. Answer : Last peak of mass spectra represent the molecular ion, where 3 (12) + 6 (1) + 1 (16) = 58 5.b) Candidate was able to explain the fragment ion representing each m/e, and was able to deduce the structure of X 5.c) Candidate can deduce the m/e for 14, 26, 27, 42 and 58 accordingly, based on the structure drawn.

(a) What is the molecular formula of X? Explain your

answer. [2]

(b) Identify the ionic species responsible for peaks

at relative masses 15 and 43. What is the

structural formula of X? [3]

(c) For each of the other lines, identify the ions that

are responsible for their occurrence. [5]

PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry

This document is not under copyrights of LRT Documents. The questions above is not printed question.4 of 6

Page 5: C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

6. The diagram below shows one type of mass spectrometer

Comment by examiner : 6.a) Candidate explained the processes that occur in each part labeled A to F correctly, and applied suitable equation take place in ionization chamber. Full marks were rewarded if candidate was able to explain accordingly.

Comment by examiner : 6.b) Out of 5 line, only 4 were answered correctly. The m/e at 83 is actually correspond to 2H-81Br+ since the mass of H is 2 while Br is 81. Most of the student will miss concept, thinking that 83 is another isotope. 6. c) Since Br2+ is at 162, if it were brought to charge 2nd time, Br22+ shall have the m/e at 162 / 2 = 81.

(a) Identify the functions or processes that occur in each part

labelled A to F. A mixture of 2H2 and 81Br2 was analysed by

using the spectrometer. The pattern of lines caused by ions

with single charges was obtained as follows [6]

(b) State the ions that caused each of the following lines. 2,

4, 81, 83, and 162 [3]

(c) What is the nucleon number registered on the scale of

that instrument if the heaviest ion obtained a second

charge? [1]

PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry

This document is not under copyrights of LRT Documents. The questions above is not printed question.5 of 6

Page 6: C1 - Atoms, Molecules and Stoichiometry [Structure Question] ANSWER

7. The diagram below shows ionic distances in a mass spectrometer using bromine gas. (Bromine has a

relative atomic mass of 79.91 and it contains two isotopes with nucleon numbers of 79 and 81)

Each of the groups of lines A, B, and C was produced by one of these ions : Br+ (g), Br2+ (g), and

Br2+(g). State which ion produces the lines in Group A Group B Group C

Identify each line in Groups B and C, and label the line in each group that has the highest intensity.

Comment by examiner : 7.a) Candidate seems to misun-derstood the peak and deflection of Group A and C. 2 factors which determine the deflection are mass and charge of a substance. In order to deflect further, an ion must has high mass and low charge. So for Group A, since it deflect the shortest, so it must have a high charge, while Group C deflect the furthest, so it must has high mass and loe charge Answer : Group A : Br2+ (g) Group C : Br2+ (g) Based on the explanation above, for each line at Group B and C, the m/e of each peak should be B1 : 79 Br +

B2 : 81 Br +

C1 : (79 Br – 79 Br ) +

C2 : (79 Br – 81 Br ) +

C3 : (81 Br – 81 Br ) +

PPU 962 Chemistry PPU 962 Chemistry: SEM 1, Chapter 1 = Atoms, Molecules and Stoichiometry

This document is not under copyrights of LRT Documents. The questions above is not printed question.6 of 6