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Atoms,
Molecules
and
Stoichiometry
Contents
1. MPM Specimen Paper2. STPM 2013 Semester 1
3. STPM 2014 Semester 1
4. Malaysia Matriculation Paper 2013
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Objective questions
Q1 ( MPM Specimen Paper )
\
Answer : D
Explanation
Particle Number of protons Number of
electrons
Number of
neutrons
7 7 9
2- 8 10 10
9 9 10
+ 35 34 44
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Question 2 ( MPM Specimen Paper )
Answer : D
Explanation
RAM = ( )( )
= 14.00
RMM of X2is 28 . Isotopes of X have same number of protons but different in number of
neutrons .
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Question 3 ( STPM 2013 Sem 1 )
Tetraethyllead(IV) , (CH3CH2)4Pb , is added to petroleum as an antiknock agent .
(CH3CH2)4Pb is prepared from ethane , CH3CH3, according to the following
reactions :
CH3CH3+ Cl2 CH3CH2Cl + HCl
4CH3CH2Cl + 4NaPb (CH3CH2)4Pb + 3Pb + 4NaCl
Which statement about the reactions is not true ?
A. One mole of Cl2produces one mole of CH3CH2Cl .
B. Four moles of Cl2produce one mole of (CH3CH2)4Pb .
C. One mole of HCl is formed for each mole of (CH 3CH2)4Pb produced .
D. Four moles of CH3CH3is required for each mole of (CH3CH2)4Pb produced .
Answer : C
Explanation
Overall equation :4CH3CH3+ 4Cl2+ 4NaPb (CH3CH2)4Pb + 3Pb + 4NaCl + 4HCl
1 mole of Cl21 mole of CH3CH2Cl 4 mole of Cl2 1 mole of (CH3CH2)4Pb 1 mole of (CH3CH2)4Pb 4 mole of HCl 4 mole of CH3CH31 mole of (CH3CH2)4Pb
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Question 4 ( STPM 2014 Sem 1 )
The proton number of ion Y is 16 , its nucleon number is 33 and it has the same
number of electrons and neutrons . What is the charge of ion Y ?
A. -2
B. -1
C. +1
D. +2
Answer : B
Explanation
Nucleon
number
Number of
protons
Number of
electrons
Number of
neutrons
33 16 17 17
Since Y has one electron more than proton , thus it carries a charge of -1 .
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Question 5 ( STPM 2014 Sem 1 )
An element X with a relative atomic mass of 28.1 consists of isotopes28
X ,29
X and
30X . If the percentage abundance of
29X and
30X are the same , what is the
percentage abundance for28
X ?
A. 0.32
B. 3.33
C. 93.3
D. 96.7
Answer : C
Explanation
Let the percentage abundance of 29X and 30X be a . Let the percentage abundance of 28X be ( 100aa ) . RAM = 28.1 () () ()
= 28.1
a = 3.33 %
Percentage abundance of 29X and 30X = 3.33 % Percentage abundance of 28X= (1003.333.33 )%
= 93.34 %
= 93.3 %
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Structured & Essays Question
Question 1 ( MPM Specimen Paper )
(a) (i) Name the subatomic particle Y . [1m]
Answer : neutron(ii) Draw the paths of the beams of electrons & hydrogen ions in the above
diagram . [2m]
Answering skill : Beam of electrons deflect more than that of H+ions .
Answer :
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(iii) If a beam of deuterium ions is passed through the electric field , explain the
difference in deflection angle between the beam of hydrogen ions and that of
deuterium ions .[2m]
Answer :
The deflection angle for deuterium ions is smaller than that of hydrogen ions . This is because deuterium ion has a larger m/e value than hydrogen ions .* Alternative answer :
The deflection angle for deuterium ion is half of that for hydrogen ion . This is because the deuterium ion is twice as heavy as hydrogen ions .
* Extra notes
The heavier the ions ( larger m/e value ) , the smaller the deflection angle forthat ion .
Isotopes for hydrogenIsotopes Nucleon
number
Number of
protons
Number of
electrons
Number of
neutrons
( protium )
1 1 1 0
( deuterium )
2 1 1 1
( tritium )
3 1 1 2
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(b) P+and Q
-ions are isoelectronic with the
isotope .
(i) State the nucleon number of the isotope .
Answer : 40(ii) Identify P
+& Q
-ions .
Answer :~ P
+ion : K
+ion / potassium ions
~ Q-ion : Cl
-ion / chloride ions
*Extra Note
Isoelectronic species : species with same number of electrons For P+ions :
Number of
protons
Number of
electrons
19 18
For Q-ions :Number of
protons
Number of
electrons
17 18
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(c) If the percentage of the actual yield of FeCl3obtained in the laboratory is 82.0% ,
calculate the mass of FeCl3produced . [2m]
Answer :
Mass of FeCl3produced = 497.8 82.0%= 408.2 g
(d) Explain why there is a difference between the calculated theoretical value yield
of FeCl3 with that of the actual yield obtained in the laboratory . [2m]
Answer :
Some of the chlorine gas escapes from the reacting vessel . Some of FeCl3will be decomposed and forms FeCl2.
2 FeCl3(s) 2 FeCl2(s) + Cl2(g)
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Question 3 ( Matriculation 2013 )
Silicon exists as three stable atoms with nucleon number of 28 , 29 and 30 . The
proton number of silicon is 14 .
(a) Write the symbol for any one of 3 silicon atoms . Indicating its proton and
nucleon number in the symbol . [1m]
Answer :
@ @ (b) If the amount of a sample of silicon with nucleon number of 28 , 29 and 30 are
92.2% , 4.68% and 3.12% , calculate the relative atomic mass of silicon . [2m]
Answer :
RAM = ( )( )( )
= 28.1
(c)Sketch and label the mass spectrum of silicon . [2m]
Answer :
Answer :
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Question 4 ( Matriculation 2013 )
When 80.0g of ethene gas , C2H4 reacts with 110.0 g of hydrochloric acid , HCl ,
chloroethane , C2H5Cl is produced .
(a) Write a balanced chemical equation for the reaction . [1m]
Answer : C2H4(g) + HCl (aq) C2H5Cl (g)(b) Determine the limiting reactant in the reaction . [3m]
Answer :
Number of moles of ethene =
(
) ( )
mol
= 2.86 mol
Number of moles of HCl =
mol
= 3.01 mol
Since the number of moles for ethene is less than that for hydrochloric acid ,thus the limiting reactant is ethene gas .
(c) Calculate the mass of C2H5Cl produced . [3m]
Answer :
Number of moles of C2H5Cl = number of moles of ethene gas= 2.86 mol
Mass of C2H5Cl produced = 2.86 [ 2(12.0) + 5(1.0) + 35.5 ]
= 184 g ( 3s.f )
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(d) Calculate the mass of excess reactant remained at the end of the reaction . [3m]
Answer :
Number of HCl used = number of moles of ethene gas= 2.86 mole
Number of moles of HCl remained = 3.01 - 2.86= 0.15 mol
Mass of excess HCl remained = 0.15 ( 35.5 + 1.0 )= 5.48 g ( 3sf )d
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