NCERT Exemplar Solutions Class 9 Science Atoms and Molecules

Multiple Choice Questions (MCQs).................

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)

Solution 1:   (d) (ii) and (iv).

The options (ii) and (iv) reflect 360 g of water correctly.
From (ii) point,

The 1 mole of water = molar mass of water = 18 g

20 moles of water = 18 g × 20 = 360 g

From (IV) 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

Solution 2:   (d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch. 

This statement is incorrect. As the molecules and ions aggregate in large numbers together to form the matter, the correct statement is. With our eyes, we can't see the individual molecules/ions, but we can see the different substances that are a large collection of molecules/ions. So, choice (d) is wrong.
Most elements' atoms are chemically reactive and do not exist in their free state. The atoms of only noble gases reside in the Free State and are chemically unreactive. In general, atoms exist in two forms: I molecules and (ii) ions.

As a consequence, atoms are the essential building blocks of molecules and ions. Since atoms obtain the stable noble gas electron arrangement when they form molecules or ions, they become stable. As a consequence, they have a neutral disposition.

Question 3:  The chemical symbol for nitrogen gas is
(a) Ni                                    (b) N₂                          (c)N⁺                         (d) N

Solution 3:   (b) N₂

Since nitrogen is a diatomic compound, it exists as N2 molecules.

Question 4:  The chemical symbol for sodium is

(a) So                (b) Sd                (c) NA                  (d) Na

Solution 4:   (d) Na.

The chemical symbol for sodium is ‘Natrium,' which is derived from its Latin name. The first letter in a ‘two letter' symbol is the ‘capital letter,' while the second letter is the ‘small letter.' As a consequence, its symbol is ‘Na.'

Question 5:  Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C₁₂ H₂₂ O₁₁)                           (b) 2 moles of C0₂
(c) 2 moles of CaC0₃                                                        (d) 10 moles of H₂0

Solution 5:   (c) 2 moles of CaC0₃ .

(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) = 100g
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

Solution 6:   (d) 18 g of CH₄  

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

Solution 7:   (c) 1 g H₂ 

As,                

Question 8:  Mass of one atom of oxygen is -

Solution 8:   (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

Mass of one atom of oxygen =    16 /6.023 X 10²³  g

Question 9:  3.42 g of sucrose is dissolved in 18g of water in a beaker. The number of oxygen atoms in the solution is-
(a) 6.68 x 10²³          

(b) 6.09 x10²²           

(c) 6.022 x10²³         

(d)6O22 x10²¹

Solution 9:   (a) 6.68 x 10²³ 

Step 1: Molar Mass of sucrose, C₁₂H₂O₁₁ = 12 x 12 + 1 x 22 + 16 x 11 = 342g

            Or 342g = 1 mole of sucrose

            3.42g = 0.01 mole of sucrose

1 mole of sucrose (C₁₂H₂O₁₁) contain O-atoms = 11 x 6.022 x10²³ atoms

 0.01 mole of sucrose will contain O – atoms

= 0.01 x 11 x 6.022 x 10 ²³ atoms = 6.6242 x 10 ²²

Step 2: 18 g of water (H₂O) = 1 mole of water

                1 mole of water (H₂O) contains O – atoms = 6.022 x10²³ atoms

Step 3: By adding the number of O – atoms present in 3.42g of sucrose and 18g of water,

                6.022 x10²³ + 6.6242x10²² = 10²² (60.22 + 6.6242)

= 66.844 x10²² = 6.68 x10²³ atoms

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

Solution 10:   (c) when energy is either given to, or taken out from the system. 

When energy is either given to or taken away from a system, it may cause a change in its physical state. Since energy changes the magnitude of attraction forces between particles, it aids in changing the physical states of matter (i.e., solid, liquid, and gas).

Question 11:  Which of the following represents a correct chemical formula? Name it.
(a)CaCl            (b) BiP0₄         (c) NaS0₄            (d) NaS

Solution 11:   (b) BiP0₄         

Bismuth phosphate, or BiP0₄, is the proper formula.

Short Solution Type Questions................

Question 12:  Write the molecular formulae for the following compounds

(a)Copper (II) bromide                            

(b) Aluminum (III) nitrate

(c) Calcium (II) phosphate                      

(d) Iron (III) sulphide

(e) Mercury (II) chloride                          

(f) Magnesium (II) acetate

Solution 12:

(a)Copper (II) bromide = CuBr₂                         

(b) Aluminum (III) nitrate = AI(NO₃)³

(c) Calcium (II) phosphate = Ca(PO₄)₂                

(d) Iron (III) sulphide = Fe₂S₃

(e) Mercury (II) chloride = HgCI₂                   

(f) Magnesium (II) acetate = (CH₃COO)₂Mg

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

                   Cu²⁺  ,  Na⁺  ,  Fe³⁺  ,   Cl^- ,    SO^2-₄  ,   PO^3-₄

Solution 13:

Compounds of Cu²⁺ with

(i)            Cl^-  CuCI₂

(ii)           SO^2-₄   CuSO₄

(iii)          PO^3-₄  Cu₃(PO₄)₂

Compounds of Na⁺ with

(i)            Cl^-  NaCI

(ii)           SO^2-₄  Na₂ SO₄

(iii)          PO^3-₄  Na₃PO₄

Compounds of Fe³⁺ with

(i)            Cl^-  FeCI₃

(ii)           SO^2-₄  Fe₂(SO₄)₃

(iii)          PO^3-₄  FePO₄

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

Solution 14:

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

Solution 15:

Question 16:  Which of the following symbols of elements are incorrect? Give their correct symbols.
(a) Cobalt CO                                               (b) Carbon c
(c) Aluminum AL                                       (d) Helium He
(e) Sodium So

Solution 16:

(a) The symbol ‘CO’ for cobalt is wrong. The right symbol for it is Co.

(b) The symbol ‘c’ for carbon is wrong. The right symbol for it is C.

(c) The symbol ‘AL’ for aluminum is wrong. Al is the proper symbol for it.

(e) The symbol ‘So’ for sodium is incorrect. The right symbol for it is Na. (It comes from the Latin word 'Natrium'.)

(d) ‘He’ is the right symbol for helium.

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) Aluminum fluoride
(e) Magnesium sulphide

Solution 17:

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

a)      CO₃^2-

b)      PO₄^3-

c)       P₂O₅

d)      CO 

Solution 18:
(a) Number of atoms in CO₃^2- = number of C-atoms + number of O-atoms = 1 + 3 = 4
(b) Number of atoms in PO₄^3- = number of P-atoms + number of O-atoms = 1 + 4 = 5
(c) Number of atoms in P₂O₅ = number of P-atoms + number of O-atoms = 2 + 5 = 7
(d) Number 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?

Solution 19:   In water molecule (H₂0),

Number of neutrons = [(number of neutrons in H) x 2 + (number of neutrons in O)]
                                      = 0 x 2 + 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

Solution 20:   Yes, a substance's solubility varies with temperature. Solubility refers to the overall volume of a solute that can be dissolved in 100 g of a solvent at a given temperature.

Temperature has an effect on solubility

i)        Solids' solubility in liquids typically increases as the temperature rises and decreases as the temperature falls.

ii)       The solubility of gases in liquids usually decreases as the temperature rises and rises as the temperature falls.

Let's look at copper sulphate as an example.

Below are the solubility’s of copper sulphate in water at different temperatures.

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

Solution 21:  On the basis of their atomicity, they are graded as follows: 

i)        Monoatomic: - (k) He, (1) Ag

ii)       Diatomic: -        (a) F₂, (i) HCl

iii)     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 colored powder which is either sugar or salt. How would you identify it without testing?

Solution 22:   Without testing, we can distinguish between sugar and salt in a number of ways.

i)        Separately dissolve the salt and sugar in the alcohol; the sugar will dissolve while the salt will not.

ii)       Heat the salts separately; sugar melts easily, but salt does not.

iii)     Separately dissolve these two in water. Because of the presence of the Na+ and CF ions in salt solution, it conducts electricity, whereas sugar solution does not. Simply use an ohmmeter to measure a drop of the solution and you'll be able to say the difference right away.

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.

Solution 23:   Given that molar atomic mass of Mg = 24g mol^-1

24g of Mg = 1 mol

12 g of Mg =  1x12/24=  = 0.5 mol

Long Solution 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.

Solution 24:

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

Solution 25:  

a)      CaCO₃ → Ca: C :O = 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 : O = 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 : O : 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.

Solution 26:

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.

Solution 27:   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.

Solution 28:   Given that, molar mass of Hg = 200.6 g mol ^-1

And that of         S = 32 g mol ^-1

Molar mass of HgS = molar mass of the Hg + molar mass of the S

Molar mass of HgS = 200.6 + 32 = 232.6g

232.6g of HgS contains Hg = 200.6 [as molar mass of Hg = 200.6g mol^-]

225 g of HgS will contains Hg = (200.6× 225)/232.6 = 194.05g

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?

Solution 29:   Given that, mass of one steel screw = 4.11g

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?

Solution 30:   Given that, number of oxygen atoms in the given sample = 2.58 ²⁴

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?

Solution 31:

a)      1 mole = molar mass of a substance.
∴ 1mole of carbon atoms weight = 12g
∴ the weight of 5 moles of carbon atoms= 12 x 5 = 60 g
As a consequence, Raunak's container has a weight of 60 g.
∴ 1 mole of sodium atoms weigh =23g
∴ The weight of 5 moles of sodium atoms= 23 x 5 = 115 g
As a result, Krish's container weighs= 115 g
As a result, Krish's container is heavier than Raunak's
As, 1 mole = 6.022 x 10²³ atoms

b)      Because both containers contain 5 moles of carbon and sodium, they have the same number of atoms, i.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

Solution 32:

For Species property H₂O

Number of moles = 2 and mass – 36 g

Number of particles = number of moles x 6.022 x 10²³

                        = 2 x 6.022 x 10²³ = 1.2044 x 10²⁴

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

Solution 38:

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? 

Solution 39:

Question 40:  What are ionic and molecular compounds? Give examples.

Solution 40:   Ionic compounds are those that are made up entirely of ions. Powerful electrostatic forces of attraction, known as ionic bonds or electrovalent bonds, bring positively charged ions (cations) and negatively charged ions (anions) together in ionic compounds, such as sodium chloride (NaCI), calcium oxide (CaO), and others.
Compounds in which the atoms of the elements share electrons through covalent bonds are known as molecular compounds. Covalent bonds are found in these compounds, such as methane (CH₄), water (H₂0), and so on. 

Question 41:  Compute the difference in masses of one mole each of aluminum atoms and one mole of its ions (Mass of an electron is 9.1 x 10^-28 g). Which one is heavier?

Solution 41:

Ionization 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 are:
= 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.

Solution 42:

Mass of silver (Ag) ornament = mg

Mass of gold used for polishing =  mg = 0.01 mg

Atomic mass of Ag = 108 u

 

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?

Solution 43:

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 are___________

d)      Formula of sodium carbonate is ___________.. and that of ammonium sulphate is _______

Solution 44:

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 are 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.

Solution 45:

  

 

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

1. Cl
2. H
3. Ar
4. O
5. Xe
6. N
7. He
8. F
9. Kr
10. Rn
11. Ne

(b) Identify the total number of inert gases, their names and symbols from this crossword puzzle. 

Solution 46: (a) Name of the elements are as follows:

1. Cl -  Chlorine
2. H -  Hydrogen
3. Ar - Argon
4. O -  Oxygen
5. Xe - Xenon
6. N -  Nitrogen
7. He - Helium
8. F   -  Fluorine
9. Kr -  Krypton
10. Rn -  Radon
11. Ne -  Neon

 

 

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

Solution 47:

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 th  e volume of water so consumed assuming the density of water to be 1 g cm^-3.

Solution 48:  During photosynthesis, following reaction takes place