Page 19 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 6

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The volume of 50 drops of carbon tetrachloride, CCl 4 is V cm . Each drop of CCl 4
contains n molecules. If the density of CCl 4 is 1.60 g cm , what is the expression
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for the Avogadro’s constant? [Relative molecular mass of CCl 4 is 154.]
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1.60 × 50 × n 154 × 50 × n
A ————–—– C ————–—–
154 × V 1.60 × V
1.60 × 50 × V 154 × 50 × V
B ————–—– D ————–—–
154 × n 1.60 × n
Answer: C V
Term
1 Volume of 1 drop = —–
50
V
Volume of n molecules = —–
50
V
50
Volume of 1 mole of molecules = —– ÷ n × N A
Mass of 1 mole of molecules = 154 g
Mass
Since density = ———–
Volume
Mass of 1 mole of molecules = Density × Volume
V
154 = 1.60 × —– × —–
N A
50 n
154 × n × 50
N A = —–———––
1.60 × V
Question 7

Using chlorine as an example, which of the following formulae can be used to
determine Avogadro’s constant?
Mass of 1 mol gas Volume of 1 mol gas
A ————–——–—– C ————–——–—––
Mass of 1 atom Mass of 1 atom
Mass of 1 mol gas Density of 1 mol gas
B ————–——–—– D ————–——–——–
Mass of 1 molecule Density of 1 molecule

Answer: B
Mass of 1 mol Cl 2 gas = m g Volume of 1 mol Cl 2 gas =V
m
V
Mass of 1 Cl 2 molecule = —– Volume of 1 Cl 2 molecule = —–
N A N A
m
V
Mass of 1 Cl atom = —– – Volume of 1 Cl atom = —––
2N A 2N A

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01 STPM Q&A Chem T1 layout.indd 14 4/19/19 9:28 AM