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Atoms and Molecules QA Test (1)


Question 1 ( 1.0 marks)
Calculate the mass (in grams) of the iron sulphide formed when 5.6 g of iron combines with 3.2 g of sulphur.
(Atomic mass of Fe = 56 u and Atomic mass of S = 32 u)
Solution:
Fe + S → FeS
56 g of Fe + 32 g of S = 88 g of FeS
∴5.6 g of Fe + 3.2 g of S = 8.8 g of FeS
Question 2 ( 1.0 marks)
What is the mass of 0.5 mole of Cl2 gas?
Solution:
The molecular mass of chlorine is 35.5 u.
Mass = Molar mass × Number of moles
= 35.5 × 2
= 71 g
Question 3 ( 1.0 marks)
Define one mole of a substance.
Solution:
One mole of a substance is the quantity of the substance containing 6.022 × 1023 molecules of particles.
Question 4 ( 2.0 marks)
Calculate the molecular mass of Ca (OH)2.
Solution:
Molecular mass of Ca (OH) 2 = (1 × 40 + 2 × 16 + 2 × 1) g
= (40 + 32 + 2) g = 74 g
Question 5 ( 2.0 marks)
What is the significance of the symbols of elements?
Solution:
Significance of the chemical symbols of elements:
(i) A chemical symbol represents a particular element.
(ii) It represents one atom of that element.
Question 6 ( 2.0 marks)
Give a difference between an atom and a molecule.
Solution:
An atom is indivisible, whereas a molecule can be divided into its constituent atoms.
Question 7 ( 2.0 marks)
Calculate the mass (in grams) of 3 moles of ammonia.
Solution:
The molecular formula of ammonia is NH3.
Mass of 1 mole of ammonia = 1 × 14 + 3 × 1
= 14 + 3
= 17 g
Therefore, mass of 3 moles of ammonia = 17 × 3
= 51 g
Question 8 ( 3.0 marks)
Answer the following questions.
(a) What is atomicity?
(b) Why is He monoatomic and N diatomic?
Solution:
(a) The number of atoms that combine to form a molecule is called the atomicity of that molecule.
(b) He is monoatomic as it has one atom in its molecule. It can exist freely due to a completely filled sub-shell.
N is diatomic as it has two atoms that are chemically bonded to form a molecule.
Question 9 ( 3.0 marks)
Match the following.
Ion
Symbol
Ammonium
Fe 2+
Ferrous ion
Ferric ion
NH4+
Nitrate
H−
Nitrite
Fe3+
Hydride
Solution:
Ion
Symbol
Ammonium
NH4+
Ferrous ion
Fe 2+
Ferric ion
Fe 3+
Nitrate
Nitrite
Hydride
H−
Question 10 ( 5.0 marks)
Deduce the chemical formula of calcium hydroxide. What information do you get from the formula?
Solution:
Thus, the chemical formula of calcium hydroxide is Ca (OH)2.
We can conclude the following from this chemical formula.
(i) The elements present in Ca (OH)2 are calcium (Ca), oxygen (O) and hydrogen (H).
(ii) Ca (OH)2 represents one molecule of calcium hydroxide that contains two atoms each of hydrogen and oxygen, and one atom of calcium.
(iii) The valency of Ca is 2 and that of hydroxide is 1.
(iv) Ca (OH)2 is a neutral molecule. It has no charge.

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