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S1C1
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S1C1 Atom, Molecules & Stoichiometry
S1:03P2Q1
2. Beams of particles from different sources are passed through an electric field. The
path of the beam of sub-atomic particles Y through the electric field is shown in the
diagram below.
(a) (i) Name the sub-atomic particles Y.
(ii) Draw the paths of the beams of electrons and hydrogen ions in the above diagram.
(iii) If a beam of deuterium ions is passed through the electric field, state the difference
in deflection angle between the beam of hydrogen ions and that of deuterium ions.
[4m]
40 X
(b) P+ and Q- ions are isoelectronic with the 18 isotope,
(i)State the nucleon number of the 40 X
isotope.
18
(ii) Identify P+ and Q- ions.
1
S1C1 Atom, Molecules & Stoichiometry
S2.17Q16
16 (a) The nucleon number of an isotope of element X is 64. The isotope has 34 neutron
Calculate the number of protons in X. [1m]
Num of proton X =
=
(b) The cation derived from X has 28 electrons. Write the symbol for the cation. [1m]
Charge on ion X =
=
The ion is
(c) The mass spectrum of element X is shown below
Relative
abundance
100
56
38
8
m/e
64 66 67 68
Determine the relative atomic mass of X.[2m]
(d) An impure sample of X is treated with an excess of sulphuric acid. Write a balanced
equation for the reaction. [1m]
(e) If 0.0764 g of hydrogen is obtained from a sample of 3.86g, calculate the percentage
purity of the sample. [3m]
S1. Mol of H2 =
S2. Mass of pure H2 in sample X =
S3. % Purity = Mass of pure compound in sample x 100%
Total mass of impure sample
2
S1C1 Atom, Molecules & Stoichiometry
1.2 Relative atomic, isotopic, molecular and formula masses
S3: 2000Q4(s)
1. (a) (i) Define relative atomic mass.
(ii) The relative atomic mass of X is 30.97. By how many times is an atom of X heavier
than one atom of carbon-12?
(iii) Naturally occuring sulphur consists of four isotopes with relative abundance as given
in the table below.
Relative isotopic mass Relative abundance(%)
31.97 95.02
32.97 0.75
33.96 4.21
35.96 0.02
Calculate the relative atomic mass of sulphur.[6m]
3
S1C1 Atom, Molecules & Stoichiometry
1.2 Relative atomic, isotopic, molecular and formula masses
S3: 2000Q4(s)
1. (b) The mass spectrum of 1,2-dichloropropane shows three peaks at mass/charge
ratio of 112, 114 and 116.
(i) If 1,2-dichloropropane consists of the hydrogen-1, carbon-12，chlorine-35 and
chlorine-37 isotopes, write the formula of the ions responsible for the peaks.
(ii)If the ratio of chlorine-35 to chlorine-37 is 3:1, calculate the percentage abundance of
the peaks.
(iii) Sketch and label the mass spectrum for 1,2-dichloropropane showing the three
[10]
m/e Ion abundance(%)
37
4
S1C1 Atom, Molecules & Stoichiometry
1.1 Structure of Atom
Isotope
4. Iodine-123 is used in brain imaging and
iodine-131 is used in detecting the activity
of thyroid. Both isotopes
A are stable
B have different crystal structures
C react with aqueous solution of KI
D have different number of protons
Since iodine-123 and iodine-131 can be
used in imaging and detecting, they
must be radioactive and emit
radioactive particles/radiations. The
crystalline structure does not depend
on the type of isotope present.
Isotopes have similar chemical
reactions. Both isotopes react with
aqueous KI to form the I3- complex ion:
I2 + KI → K+ + I3- lsotopes of the same
element have the same proton number.
They differ in the nucleon number (the
number of neutrons).
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S1C1 Atom, Molecules & Stoichiometry
3. 16Q1
A solution is prepared by dissolving 14.9
g XCl in water. When excess silver nitrate
solution is added to the solution, 28.7 g of
silver chloride precipitate is formed. What
is the relative atomic mass of X?
A 11.0 B 39.0 C46.5 D 74.5
The reaction between XCl and silver
nitrate in the precipitation of AgCl
XCl(aq) + AgNO3→XNO3 + AgCl
Initial/g 14.9
Final/g 0 28.7
The mass of 1 mol of AgCl = (108 +
35.5)g =143.5 g
From the equation:
Mass of XCl required to produce 1 mol
(143.5g) of
AgCl = 143.5/28.7 x 14.9 g
=74.5 g
Mass of 1 mol of XC1 = 74.5 g
Let the Ar of X = a
(a + 35.5) = 74.5
a = 39.0
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