NCERT Exemplar Problems Class 9 Science – Atoms and Molecules

Question 1:
Which of the following correctly represents 360 g of water?
(i) 2 moles of H₂0
(ii) 20 moles of water
(iii) 022 x 10²³ molecules of water
(iv) 12044 x 10²⁵ molecules of water
(a) only (i) (b) (i) and (iv)
(c) (ii) and (iii) (d) (ii) and (iv)
Answer:
(d) (ii) and (iv) points correctly represent 360 g of water.
(ii)From point,

Question 2:
Which of the following statements is not true about an atom?
(a)Atoms are not able to exist independently
(b)Atoms are the basic units from which molecules and ions are formed
(c)Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch
Answer:
(d) This statement is not true
The correct statement is as The molecules and ions aggregate together in large number to form the matter. We cannot see the individual molecules/ions with our eyes, only we can see the various substances which are a big collection of molecules/ions. So, option (d) is incorrect.
Atoms of most of the elements are chemically very reactive and do not exist in the free state. The atoms of only noble gases are chemically unreactive and exist in free state. Atoms usually exist in the two forms
(i) molecules and (ii) ions
Therefore, atoms are the basic unit from which molecules and ions are formed.
When atoms form molecules or ions, they become stable because they acquire the stable noble gas electron arrangement. Therefore, they are neutral in nature.

Question 3:
The chemical symbol for nitrogen gas is
(a) Ni (b) N₂ (c)N⁺ (d) N
Answer:
(b) Nitrogen molecule is diatomic molecule, therefore, it exists as N₂ molecules.

Question 4:
The chemical symbol for sodium is
(a) So (b) Sd (c) NA (d) Na
Answer:
(d) The chemical symbol for sodium is derived from its Latin name ‘Natrium’. In a ‘two letter’ symbol, the first letter is the ‘capital letter’ but the second letter is the ‘small letter’. Therefore, its symbol is ‘Na’.

Question 5:
Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C₁₂ H₂₂ O,,) (b) 2 moles of C0₂
(c) 2 moles of CaC0₃ (d) 10 moles of H₂0
Answer:
(c) 2 moles of CaC0₃ would weigh the highest.
(a)Mass of 1 mole of sucrose (12 x12) + (1 x22)+ (16×11)= 342 g
Mass of 0.2 mole of sucrose = 342 x 0.2 = 68.4g
(b)Mass of 1 mole of C0₂ = 12 + (16 x 2) = 44g .-. Mass of 2 moles of C0₂ = 44 x 2 = 88 g
(c)Mass of 1 mole of CaC0₃ = 40 + 12 + (16 x 3) = 10Og
Mass of 2 moles of CaC0₃ = 100 x 2 = 200 g
(d)Mass of 1 mole of H₂0 = 2 + 16 = 18g
Mass of 10 moles of H₂0 = 18x 10 = 180g
Therefore, mass of 2 moles of CaC0₃ is the highest, i.e., 200 g.

Question 6:
Which of the following has maximum number of atoms?
(a) 18 g of H₂0 (b) 18 g of 0₂
(c) 18 g of C0₂ (d) 18 g of CH₄
Answer:

Question 7:
Which of the following contains maximum number of molecules?
(a) 1 g C0₂ (b) 1 g N₂ (c) 1 g H₂ (d) 1 g CH₄
Answer:

Question 8:

Answer:
(a) Mass of 6.023 x 10²³ atoms of oxygen = gram atomic mass of oxygen Mass of 6.023 x 10²³ atoms of oxygen = 16g

Question 9:
3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of oxygen atoms in the solution are
(a) 6.68 x 10²³ (b) 6.09 x10²² (c) 6.022 x10²³ (d)6O22 x10²¹
Answer:

Question 10:
A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
Answer:
(c) A change in the physical state can be brought about when energy is either given to, or taken out from the system. It is because, energy change helps in changing the magnitude of attraction forces between the particles, thus helps in changing the physical states (j.e., solid, liquid, gas) of matter.

Short Answer Type Questions

Question 11:
Which of the following represents a correct chemical formula? Name it.
(a)CaCl (b) BiP0₄ (c) NaS0₄ (d) NaS
Answer:
(b) BiP0₄, is the correct formula, its name is bismuth phosphate.

Question 12:
Write the molecular formulae for the following compounds
(a)Copper (II) bromide (b) Aluminium (III) nitrate
(c) Calcium (II) phosphate (d) Iron (III) sulphide
(e) Mercury (II) chloride (f) Magnesium (II) acetate
Answer:

Question 13:
Write the molecular formulae of all the compounds that can be formed by the combination of following ions.

Answer:

Question 14:
Write the cations and anions present (if any) in the following compounds.
(b) CH₃COONa (b) NaCI
(c) H₂ (d) NH₄N0₃
Answer:

Question 15:
Give the formulae of the compounds formed from the following sets of elements.
(a) Calcium and fluorine (b) Hydrogen and sulphur
(c) Nitrogen and hydrogen (d) Carbon and chlorine
(e) Sodium and oxygen (f) Carbon and oxygen
Answer:

Question 16:
Which of the following symbols of elements are incorrect? Give their correct symbols.
(a) Cobalt CO (b) Carbon c
(c) Aluminium AL (d) Helium He
(e) Sodium So
Answer:
(a) Cobalt CO is incorrect symbol. Its correct symbol is Co.
(b) Carbon c is incorrect symbol. Its correct symbol is C.
(c) Aluminium AL is incorrect symbol. Its correct symbol is Al.
(e) Sodium So is incorrect symbol. Its correct symbol is Na. (It is derived from Latin name ‘Natrium’).
(d) Helium, He is the correct symbol.

Question 17:
Give the chemical formulae for the following compounds and compute the ratio by mass of the combining elements in each one of them.
(a) Ammonia (b) Carbon monoxide
(c) Hydrogen chloride (d) Aluminium fluoride
(e) Magnesium sulphide
Answer:

Question 18:
State the number of atoms present in each of the following chemical species

Answer:
(a) Number of atoms in COg’ = number of C-atOms + number of O-atoms = 1 + 3 = 4
(b) Number of atoms in PO®^– = number of P-atoms + number of O-atoms =1+4=5
(c) Number of atoms in P₂0₅ = number of P-atoms + number of O-atoms =2 + 5 = 7
(d) Numbfer of atoms in CO = number of C-atoms + number of O-atoms =1 + 1 = 2

Question 19:
What is the fraction of the mass of water due to neutrons?
Answer:
In water molecule (H₂0), number of neutrons = [(number of neutrons in H) x 2 + (number of neutrons in O)]
= 0x2 + 8=8 (as number of neutrons in H =0)
Mass of 8 neutrons = 8 x 1.00893 = 8.07 (v mass of one neutron = 1.008934)
Molar mass of water = 1.008 x 2 + 16.0 = 18.016 u

Question 20:
Does the solubility of a substance change with temperature? Explain with the help of an example
Answer:
Yes, the solubility of a substance changes with temperature.
The maximum amount of a solute which can be dissolved in 100 g of a solvent at a specified temperature is called its solubility.
Effect of temperature on solubility
(i) The solubility of solids in liquids usually increases on increasing the temperature and decreases on decreasing the temperature.
(ii) The solubility of gases in liquids usually decreases on increasing the temperature and increases on decreasing the temperature.
Let us take an example of copper sulphate.
The solubilities of copper sulphate in water at various temperatures are given below

From the above data, it is clear that, when the temperature is increased from 0°C to 70°C, the solubility of copper sulphate in water increases from 14 g to 47g. The above data shows that the solubility of a salt increases on increasing the temperature.

Question 21:
Classify each of the following on the basis of their atomicity.
(a) F₂ (b) N0₂ (c) N₂0 (d) C₂H₆
(e) P₄ (f) H₂0₂ (g) P₄0₁₀ (h) 0₃
(i) HCl (j) CH₄ (k) He (l) Ag
Answer:
They are classified as
Monoatomic (k) He, (1) Ag
Diatomic (a) F₂, (i) HCl
Polyatomic (b) N0₂, (c) N₂0, (d) C₂H₆, (e) P₄, (f) H₂0₂, (g) P₄O₁₀, (h) 0₃, (j) CH₄.

Question 22:
You are provided with a fine white coloured powder which is either sugar or salt. How would you identify it without testing?
Answer:
There are many ways by which we can differentiate between sugar and salt without testing
(i) Dissolve salt and sugar separately in alcohol, sugar will be dissolved in it while salt will not be
dissolved.
(ii) Heat these salts separately, sugar will melt readily while salt will not.
(iii) Dissolve these two separately in water. Salt solution will conduct electricity due to the presence of Na⁺ion and CF ion while sugar solution will be a non-conductor. So, simply test a drop of the solution with an ohmmeter and you can immediately tell the difference.

Question 23:
Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12g. Molar atomic mass of magnesium is 24 g mol^-1.
Answer:

Long Answer Type Questions

Question 24:
Verify by calculating that
(a) 5 moles of C0₂ and 5 moles of H₂0 do not have the same mass.
(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3 : 5.
Answer:

Question 25:
Find the ratio by mass of the combining elements in the following compounds.
(a) CaC0₃ (b) MgCl₂ (c) H₂S0₄ (d) C₂H₅0H
(e) NH₃ (f) Ca(0H)₂
Answer:
(a) CaCOg → Ca: C :0 = 40:12 : 48= 10 : 3:12
(b) MgCI₂ → Mg : Cl = 24 : 2x 35.5 = 24 : 71
(c) H₂ S0₄→ H : S : O = 2x 1 : 32 : 4x 16 = 2 : 32 : 64= 1:16: 32
(d) C₂ H₅ OH → C : H : 0 = 2x 12 : 6x 1 : 16 = 24: 6:16= 12 : 3: 8
(e) NH₃ → N : H = 14 : 3x 1=14: 3
(f) Ca (OH)₂ → Ca : 0 : H = 40 : 2x 16 : 2x 1= 40: 32 :2 = 20 :16:1

Question 26:
Calcium chloride when dissolved in water dissociates into its ions according to the following equation.
CaCl₂(og) → Ca²⁺ (aq) + 2Cl^_ (aq)
Calculate the number of ions obtained from CaCl₂ when 222 g of it is dissolved in water.
Answer:

Question 27:
The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.
Answer:
Sodium atom loses an electron to form sodium ion,

Question 28:
Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS? Molar mass of Hg and S are 200.6 g mol^-1 and 32g mol^-1 respectively.
Answer:

Question 29:
The mass of one steel screw is 4.11 g. Find the mass of one mole of these steel screws. Compare this value with the mass of the earth (5.98 x 10²⁴ kg). Which one of the two is heavier and by how many times?
Answer:

Question 30:
A sample of vitamin C is known to contain 2.58 x 10²⁴ oxygen atoms. How many moles of oxygen atoms are present in the sample?
Answer:

Question 31:
Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight.
(a) Whose container is heavier?
(b) Whose container has more number of atoms?
Answer:
(a) 1 mole = molar mass of a substance.
∴ 1 mole of carbon atoms weigh = 12g
∴5 moles of carbon atoms will weigh = 12 x 5 = 60 g
Hence, Raunak’s container has weigh = 60 g In the same way,
∴ 1 mole of sodium atoms weigh =23g
∴ 5 moles of sodium atoms will weigh = 23 x 5 = 115 g
Hence, Krish’s container has weigh = 115 g
Thus, Krish’s container is heavier than Raunak’s container.
As, 1 mole = 6.022 x 10²³ atoms
(b)Here, both containers have 5 moles of each carbon and sodium, therefore, both containers have equal number of atoms, e., 5 x 6.022 x 10²³ atoms,
or 3.011 x 10²⁴ atoms in each.

Question 32:
Fill in the missing data in the following table

Answer:

Question 33:
The visible universe is estimated to contain 10²² stars. How many moles of stars are present in the visible universe?
Answer:

Question 34:
What is SI prefix for each of the following multiples and submultiples of a unit?

Answer:

Question 35:
Express each of the following in kilograms

Answer:

Question 36:
Compute the difference in masses of 10³ moles each of magnesium atoms and magnesium ions. (Mass of an electron =9.1 x 10-³¹ kg)
Answer:

Question 37:
Which has more number of atoms?
100 g of N₂ or 100 g of NH₃
Answer:

Question 38:
Compute the number of ions present in 5.85 g of sodium chloride.
Answer:

Question 39:
A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?
Answer:

Question 40:
What are ionic and molecular compounds? Give examples.
Answer:
Ionic compounds are those compounds which are made up of ions. In ionic compounds, the positively charged ions (cations) and negatively charged ions (anions) are held together by the strong electrostatic forces of attraction, which are known as ionic bonds or electrovalent bonds, e.g., sodium chloride (NaCI), calcium oxide (CaO), etc.
Molecular compounds are those compounds in which the atoms of the elements share electrons via covalent bonds . Thus, in these compounds, covalent bonds are present, e.g., methane (CH₄), water (H₂0) etc.

Question 41:
Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions (Mass of an electron is 9.1 x 10^-28 g). Which one is heavier?
Answer:
Ionisation of Al atom occurs as Al —> Al³⁺ + 3e“
Therefore, Al³⁺ ion is formed from Al atom by loss of 3 electrons. Difference in mass of 1 mole of Al atoms and 1 mole of Al³⁺ ions
= mass of 3 x 6.022 x 10²³ electrons
= (3 x 6.022 x 10²³) x (9.1 x 10^-28 g)
(as mass of an electron = 9.1 x 10^-28 g)
= 164.4×10^-5g = 1.644×10^-3g
1 mole of Al atoms is heavier than 1 mole of Al³⁺ ions.
It is because 1 mole of Al atom have e^– = 13 x 6.022 x 10²³ electrons and 1 mole of Al ion have e“ = 10 x 6.022 x 10²³ electrons

Question 42:
A silver ornament of mass’m’ gram is polished with gold equivalent to 1% of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.
Answer:

Question 43:
A sample of ethane (C₂H₆) gas has the same mass as 1.5 x 10²⁰ molecules of methane (CH₄). How many C₂H₆ molecules does the sample of gas contain?
Answer:

Question 44:
Fill in the blanks.
(a) In a chemical reaction, the sum of the masses of the reactants and
products remains unchanged. This is called………………
(b) A group of atoms carrying a fixed charge on them is called………………..
(c) The formula unit mass of Ca₃(P0₄)₂ is………………
(d) Formula of sodium carbonate is ……………….. and that of ammonium
sulphate is……………..
Answer:
(a) In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called law of conservation of mass.
(b) A group of atoms carrying a fixed charge on them is called polyatomic ion.
(c) The formula unit mass of Ca₃(P0₄)₂ is 310
(d) Formula of sodium carbonate is Na₂C0₃ and that of ammonium sulphate is
(NfUS0₄.

Question 45:
Complete the following crossword puzzle (Figure) by using the name of the chemical elements. Use the data given in the table following.

Answer:

Question 46:
(a) In this crossword puzzle (Figure), names of 11 elements are hidden. Symbols of these are given below. Complete the puzzle.

Answer:

Question 47:
Write the formulae for the following and calculate the molecular mass for each one of them.
(a) Caustic potash (b) Baking powder (c) Lime stone
(d) Caustic soda (e) Ethanol (f) Common salt
Answer:

Question 48:
In photosynthesis, 6 molecules of carbon dioxide combine with an equal number of water molecules through a complex series of reactions to give a molecule of glucose having a molecular formula C₆H₁₂0₆. How many grams of water would be required to produce 18 g of glucose? Compute the volume of water so consumed assuming the density of water to be 1 g cm^-3.
Answer: