Science Ncert_exemplar 2019 Solutions for Class 9 Science Chapter 3 Atoms And Molecules are provided here with simple step-by-step explanations. These solutions for Atoms And Molecules are extremely popular among Class 9 students for Science Atoms And Molecules Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Science Ncert_exemplar 2019 Book of Class 9 Science Chapter 3 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Science Ncert_exemplar 2019 Solutions. All Science Ncert_exemplar 2019 Solutions for class Class 9 Science are prepared by experts and are 100% accurate.

#### Question 1:

Which of the following correctly represents 360g of water?
(i) 2 moles of H2O
(ii) 20 moles of water
(iii) 6.022 × 1023 molecules of water
(iv) 1.2044 ×1025 molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)

Hence, the correct answer is option d.

#### 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

Atoms of inert gases like neon and argon exist independently.
Hence, the correct answer is option a.

#### Question 3:

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

Nitrogen exists as a diatomic molecule and the symbol is N2.
Hence, the correct answer is option b.

#### Question 4:

The chemical symbol for sodium is
(a) So
(b) Sd
(c) NA
(d) Na

The symbol for sodium is Na.
Hence, the correct answer is option d.

#### Question 5:

Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C12 H22 O11)
(b) 2 moles of CO2
(c) 2 moles of CaCO3
(d) 10 moles of H2O

Hence, the correct answer is option c.

#### Question 6:

Which of the following has maximum number of atoms?
(a) 18g of H2O
(b) 18g of O2
(c) 18g of CO2
(d) 18g of CH4

Hence, the correct answer is option d.

#### Question 7:

Which of the following contains maximum number of molecules?
(a) 1g CO2
(b) 1g N2
(c) 1g H2
(d) 1g CH4

Hence, the correct answer is option c.

#### Question 8:

Mass of one atom of oxygen is
(a) $\frac{16}{6.023×{10}^{23}}\mathrm{g}$

(b) $\frac{32}{6.023×{10}^{23}}\mathrm{g}$

(c) $\frac{1}{6.023×{10}^{23}}\mathrm{g}$

(d) 8u

Hence, the correct answer is option a.

#### Question 9:

3.42 g of sucrose are dissolved in 18 g of water in a beaker. The number of oxygen atoms in the solution are
(a) 6.68 × 1023
(b) 6.09 × 1022
(c) 6.022 × 1023
(d) 6.022 × 1021

Hence, the correct answer is option a.

#### 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

A change in the physical state can be brought about when energy is either given to or taken out of the system. For example, during boiling of water heat energy is given out whereas in the case of melting of ice heat is taken by the system. The change in the state of matter occurs because of energy exchange between the system and the surroundings.

#### Question 11:

Which of the following represents a correct chemical formula? Name it.
(a) CaCl
(b) BiPO4
(c) NaSO4
(d) NaS

(a) Valency of Ca is 2 and Cl is 1. The correct formula is CaCl2.
(b) Valency of Bi is 3 and PO4 is 3. The correct formula is BiPO4.
(c) Valency of Na is 1 and SO4 is 2. The correct formula is Na2SO4.
(d) Valency of Na is 1 and S is 2. The correct formula is Na2S.
Hence, the correct answer is option b.

#### 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

(a) CuBr2
(b) Al(NO3)3
(c) Ca3(PO4)2
(d) Fe2S3
(e) HgCl2
(f) Mg(CH3COO)2

#### Question 13:

Write the molecular formulae of all the compounds that can be formed by the combination of following ions

Cations: Cu2+,  Na+, Fe3+
Anions: Cl-, SO42-, PO43-

CuCl2, CuSO4, Cu3(PO4)2
NaCl, Na2SO4, Na3PO4
FeCl3, Fe2(SO4)3, FePO4

#### Question 14:

Write the cations and anions present (if any) in the following compounds
(a) CH3COONa
(b) NaCl
(c) H2
(d) NH4NO3

(a) CH3COO- and Na+
(b) Na+ and Cl-
(c) No ions are present in H2
(d) NH4+ and NO3-

#### 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

(a) CaF2
(b) H2S
(c) NH3
(d) CCl4
(e) Na2O
(f) CO
(g) CO2

#### 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

(a) Cobalt Co
(b) Carbon C
(c) Aluminium Al
(d) Helium He
(e) Sodium Na

#### 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. (You may use appendix-III).
(a) Ammonia
(b) Carbon monoxide
(c) Hydrogen chloride
(d) Aluminium fluoride
(e) Magnesium sulphide

 Chemical Formula Ratio by mass of combining atoms Ammonia (NH3) N(1): H(3) [14:3] Carbon monoxide (CO) C(1): O(1) [3:4] Hydrogen chloride (HCl) H(1): Cl(1) [1:35.5] or [2:71] Aluminium fluoride (AlF3) Al(1): F(3) [9:19] Magnesium sulphide (MgS) Mg(1): S(1) [3:4]

#### Question 18:

State the number of atoms present in each of the following chemical species

(a) ${\mathrm{CO}}_{3}^{2-}$

(b) ${\mathrm{PO}}_{4}^{3-}$

(c) P2O5

(d) CO

#### Question 19:

What is the fraction of the mass of water due to neutrons?

#### Question 20:

Does the solubility of a substance change with temperature? Explain with the help of an example.

Yes, the solubility of a substance changes with temperature. It generally increases with temperature. For example, more sugar can be dissolved in hot water as compared to cold water.

#### Question 21:

Classify each of the following on the basis of their atomicity.

 (a) F2 (b) NO2 (c) N2O (d) C2H6 (e) P4 (f) H2O2 (g) P4O10 (H) O3 (i) HCl (j) CH4 (k) He (l) Ag

Monoatomic: He, Ag
Diatomic: F2, HCl
Polyatomic with atomicity > 2
Atomicity (3) = NO2, N2O, O3
Atomicity (4) = P4, H2O2
Atomicity (5) = CH4
Atomicity (8) = C2H6
Atomicity (14) = P4O10

#### Question 22:

You are provided with a fine white coloured powder which is either sugar or salt. How would you identify it without tasting?

Salt and sugar can be differentiated from each other, by heating or dissolving in water.
(i) Sugar powder is charred to black on heating, whereas salt do not change.
(ii) A salt solution conducts electricity due to the formation of ions in the solution, whereas a sugar solution does not do so.

#### Question 23:

Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of magnesium is 24g mol–1.

The atomic mass of Mg = 24 g/mol
24 grams of Magnesium = 1 mol

#### Question 24:

Verify by calculating that
(a) 5 moles of CO2 and 5 moles of H2O do not have the same mass.
(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3:5.

#### Question 25:

Find the ratio by mass of the combining elements in the following compounds. (You may use Appendix-III)

 (a) CaCO3 (d) C2H5OH (b) MgCl2 (e) NH3 (c) H2SO4 (f) Ca(OH)2

#### Question 26:

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

CaCl2 (aq) → Ca2+ (aq) + 2Cl– (aq)

#### 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.

Number of electrons in Na atom = 11
Number of electrons in Na+ = 10
The difference of the number of electrons for 1 mole of Na atom and Na+ = 1 mole
For 100 moles the difference will be 100 moles of electrons
Mass of 100 moles of electrons= 5.48002 g

#### 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 32 g mol–1 respectively.

The molar mass of HgS = 200.6 + 32 = 232.6 grams
Mass of Hg in 232.6 of HgS = 200.6 grams

#### Question 29:

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

Mass of 1 screws = 4.11 grams

#### Question 30:

A sample of vitamic C is known to contain 2.58 ×1024 oxygen atoms. How many moles of oxygen atoms are present in the sample?

#### 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?

#### Question 32:

Fill in the missing data in the Table 3.1

Table 3.1

 Species H2O CO2 Na atom MgCl2 Property No. of moles 2 — — 0.5 No. of particles — 3.011×1023 — — Mass 36g — 115 g —

 Species H2O CO2 Na atom MgCl2 Property No. of moles 2 0.5 5 0.5 No. of particles 1.2044× 1024 3.011×1023 5×6.022×1023 = 3.011×1023 0.5×6.022×1023×3 = 9.033×1023 Mass 36 grams 22 grams 115 grams 47.5 grams

#### Question 33:

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

#### Question 34:

What is the SI prefix for each of the following multiples and submultiples of a unit?
(a) 103
(b) 10–1
(c) 10–2
(d) 10–6
(e) 10–9
(f) 10–12

#### Question 35:

Express each of the following in kilograms
(a) 5.84 × 10–3 mg
(b) 58.34 g
(c) 0.584 g
(d) 5.873 × 10–21 g

#### Question 36:

Compute the difference in masses of 103 moles each of magnesium atoms and magnesium ions.
(Mass of an electron = 9.1 × 10–31 kg)

#### Question 37:

Which has more number of atoms?
100g of N2 or 100 g of NH3

#### Question 38:

Compute the number of ions present in 5.85 g of sodium chloride.

#### 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?

#### Question 40:

What are ionic and molecular compounds? Give examples.

An ionic compound is a compound formed due to the transfer of electrons between the reacting species.  An ionic compound contains a cation which is a positive ion and an anion which is a negative ion. For example, sodium chloride is an ionic compound made up of  Na+ and Cl- ions. On the other hand, a molecular compound is a compound that is formed by sharing of electrons. It mainly consists of molecules. For example, ammonia (NH3), carbon dioxide (CO2).

#### 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 × 10–28 g). Which one is heavier?

#### 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.

#### Question 43:

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

#### 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 (PO4)2 is ————.
(d) Formula of sodium carbonate is ———— and that of ammonium sulphate is ————.

(a) Law of conservation of mass
(b) Polyatomic ion

(d) Na2CO3, (NH4)2SO4

#### Question 45:

Complete the following crossword puzzle Figure by using the name of the chemical elements. Use the data given in Table 3.2.

Table 3.2

 Across Down 2. The element used by Rutherford during his α–scattering experiment 1. A white lustrous metal used for making ornaments and which tends to get tarnished black in the presence of moist air 3. An element which forms rust on exposure to moist air 4. Both brass and bronze are alloys of the element 5. A very reactive non-metal stored under water 6. The metal which exists in the liquid state at room temperature 7. Zinc metal when treated with dilute hydrochloric acid produces a gas of this element which when tested with burning splinter produces a pop sound. 8. An element with symbol Pb

1. Silver
2. Gold
3. Iron
4. Copper
5. Phosphorous
6. Mercury
7. Hydrogen

#### Question 46:

In this crossword puzzle Figure, names of 11 elements are hidden. Symbols of these are given below. Complete the puzzle.

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

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

ure(a) 1. Chlorine

2. Hydrogen
3. Argon
4. Oxygen
5. Xenon
6. Nitrogen
7. Helium
8. Fluorine
9. Krypton
11. Neon

(b) Argon, Xenon, Helium, Krypton and Neon are noble gases.

#### 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

(a) Caustic Potash, KOH = (39+16+1) = 56 g/mol
(b) Baking powder, NaHCO3 = (23 +1+12+48)= 84 g/mol
(c) Limestone, CaCO3 = (40+12+48) =100 g/mol
(d) Caustic soda, NaOH = (23+16+1) = 40 g/mol
(e) Ethanol, C2H5OH = (24 + 6 + 16) = 46 g/mol
(f) Common salt, NaCl = (23 + 35.5) = 58.5 g/mol

#### 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 C6 H12 O6. 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.