{"id":23717,"date":"2021-05-27T15:16:06","date_gmt":"2021-05-27T09:46:06","guid":{"rendered":"https:\/\/ncertsolutions.guru\/?p=23717"},"modified":"2021-05-27T15:16:06","modified_gmt":"2021-05-27T09:46:06","slug":"ncert-solutions-for-class-9-science-chapter-3","status":"publish","type":"post","link":"https:\/\/ncertsolutions.guru\/ncert-solutions-for-class-9-science-chapter-3\/","title":{"rendered":"NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules"},"content":{"rendered":"

These NCERT Solutions for Class 9 Science<\/a> Chapter 3 Atoms and Molecules Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.<\/p>\n

Atoms and Molecules NCERT Solutions for Class 9 Science Chapter 3<\/h2>\n

Class 9 Science Chapter 3 Atoms and Molecules InText Questions and Answers<\/span><\/h3>\n

Question 1.
\nIn a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
\nSodium carbonate + ethanoic acid ? sodium ethanoate + carbon dioxide + water
\nAnswer:
\nIn the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.
\n\"NCERT
\nMass of sodium carbonate = 5.3 g (Given)
\nMass of ethanoic acid = 6 g (Given)
\nMass of sodium ethanoate = 8.2 g (Given)
\nMass of carbon dioxide = 2.2 g (Given)
\nMass of water = 0.9 g (Given)
\nNow, total mass before the reaction = (5.3 + 6) g = 11.3 g
\nAnd, total mass after the reaction = (8.2 + 2.2 + 0.9) g = 11.3 g
\n\u2234 Total mass before the reaction = Total mass after the reaction
\nHence, the given observations are in agreement with the law of conservation of mass.<\/p>\n

Question 2.
\nHydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
\nAnswer:
\nIt is given that the ratio of hydrogen and oxygen by mass to form water is 1 : 8.
\nThen, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.
\nTherefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 \u00d7 3g = 24 g.<\/p>\n

Question 3.
\nWhich postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
\nAnswer:
\nThe postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is:
\nAtoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.<\/p>\n

\"NCERT<\/p>\n

Question 4.
\nWhich postulate of Dalton’s atomic theory can explain the law of definite proportions?
\nAnswer:
\nThe postulate of Dalton’s atomic theory which can explain the law of definite proportion is:
\nThe relative number and kind of atoms in a given compound remains constant.<\/p>\n

Question 5.
\nDefine atomic mass unit.
\nAnswer:
\nMass unit equal to exactly one-twelfth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.<\/p>\n

Question 6.
\nWhy is it not possible to see an atom with naked eyes?
\nAnswer:
\nThe size of an atom is so small that it is not possible to see it with naked eyes. Also, the atom of an element does not exist independently.<\/p>\n

Question 7.
\nWrite down the formulae of the followings:
\n(i) sodium oxide
\n(ii) aluminium chloride
\n(iii) sodium sulphide
\n(iv) magnesium hydroxide
\nAnswer:
\n(i) Sodium oxide \u2192 Na2<\/sub>O
\n(ii) Aluminium chloride \u2192 AlCl3<\/sub>
\n(iii) Sodium sulphide \u2192 Na2<\/sub>S
\n(iv) Magnesium hydroxide \u2192 Mg(OH)2<\/sub><\/p>\n

Question 8.
\nWrite down the names of compounds represented by the following formulae:
\n(i) Al2<\/sub>(SO4<\/sub>)3<\/sub>
\n(ii) CaCl2<\/sub>
\n(iii) K2<\/sub>SO4<\/sub>
\n(iv) KNO3<\/sub>
\n(v) CaCO3<\/sub>
\nAnswer:
\n(i) Al2<\/sub>(SO4<\/sub>)3<\/sub> \u2192 Aluminium sulphate
\n(ii) CaCl2<\/sub> \u2192 Calcimn chloride
\n(iii) K2<\/sub>SO4<\/sub> \u2192 Potassium sulphate
\n(iv) KNO3<\/sub> \u2192 Potassium nitrate
\n(v) CaCO3<\/sub> \u2192 Calcium carbonate<\/p>\n

Question 9.
\nWhat is meant by the term chemical formula?
\nAnswer:
\nThe chemical formula of a compound means the symbolic representation of the composition of a compound. From the chemical formula of a compound, we can know the number and kinds of atoms of different elements that constitute the compound.<\/p>\n

For example, from the chemical formula CO2<\/sub> of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.<\/p>\n

\"NCERT<\/p>\n

Question 10.
\nHow many atoms are present in a
\n(i) H2<\/sub>S molecule and
\n(ii) PO4<\/sub>3-<\/sup> ion?
\nAnswer:
\n(i) In an H2<\/sub>S molecule, three atoms are present; two of hydrogen and one of sulphur.
\n(ii) In a PO4<\/sub>3-<\/sup> ion, five atoms are present; one of phosphorus and four of oxygen.<\/p>\n

Question 11.
\nCalculate the molecular masses of H2<\/sub>, O2<\/sub>, Cl2<\/sub>, CO2<\/sub>, CH4<\/sub>; C2<\/sub>H6<\/sub>, C2<\/sub>H4<\/sub>, NH3<\/sub>, CH3<\/sub>OH.
\nAnswer:
\nMolecular mass of H2<\/sub> = 2 \u00d7 Atomic mass of H
\n= 2 \u00d7 1
\n= 2u
\nMolecular mass of O2<\/sub> = 2 \u00d7 Atomic mass of O
\n= 2 \u00d7 16
\n= 32 u
\nMolecular mass of Cl2<\/sub> = 2 \u00d7 Atomic mass of Cl
\n= 2 \u00d7 35.5
\n= 71 u
\nMolecular mass of CO2<\/sub> = Atomic mass of C + 2 \u00d7 Atomic mass of O
\n= 12 + 2 \u00d7 16
\n= 44 u
\nMolecular mass of CH4<\/sub> = Atomic mass of C + 4 \u00d7 Atomic mass of H
\n= 12 + 4 \u00d7 1
\n= 16 u
\nMolecular mass of C2<\/sub>H6<\/sub> = 2 \u00d7 Atomic mass of C+ 6 \u00d7 Atomic mass of H
\n= 2 \u00d7 12 + 6 \u00d7 1
\n= 30 u
\nMolecular mass of C2<\/sub>H4<\/sub> = 2 \u00d7 Atomic mass of C + 4 \u00d7 Atomic mass of H
\n= 2 \u00d7 12 + 4 \u00d7 1
\n= 28u
\nMolecular mass of NH3<\/sub> = Atomic mass of N + 3 \u00d7 Atomic mass of H
\n= 14 + 3 \u00d7 1
\n= 17u
\nMolecular mass of CH3<\/sub>OH = Atomic mass of C + 4 \u00d7 Atomic mass of H + Atomic mass of O
\n= 12 + 4 \u00d7 1 + 16
\n= 32 u<\/p>\n

Question 12.
\nCalculate the formula unit masses of ZnO, Na2<\/sub>O, K2<\/sub>CO3<\/sub>, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
\nAnswer:
\nFormula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
\n= 65 + 16
\n= 81 u
\nFormula unit mass of Na2<\/sub>O = 2 \u00d7 Atomic mass of Na + Atomic mass of O
\n= 2 \u00d7 23 + 16
\n= 62 u
\nFormula unit mass of K2<\/sub>CO3<\/sub> = 2 \u00d7 Atomic mass of K + Atomic mass of C + 3 \u00d7 Atomic mass of O
\n= 2 \u00d7 39 + 12 + 3 \u00d7 16
\n= 138 u<\/p>\n

Question 13.
\nIf one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?
\nAnswer:
\nOne mole of carbon atoms weighs 12 g
\n(Given)
\ni. e., mass of 1 mole of carbon atoms = 12 g
\nThen, mass of 6.022 \u00d7 1023<\/sup> number of carbon atoms = 12 g
\nTherefore, mass of 1 atom of carbon 12
\n= \\(\\frac{12}{6.022 \\times 10^{23}} \\mathrm{~g}\\)
\n= 1.9926 \u00d7 10-23<\/sup> g<\/p>\n

\"NCERT<\/p>\n

Question 14.
\nWhich has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
\nAnswer:
\nAtomic mass of Na = 23 u (Given)
\nThen, gram atomic mass of Na = 23 g
\nNow, 23 g of Na contains = 6.022 \u00d7 1023<\/sup>
\nnumber of atoms
\nThus, 100 g of Na contains \\(\\frac{6.022 \\times 10^{23}}{23} \\times 100\\)
\nnumber of atoms
\n= 2.6182 \u00d7 1024<\/sup> number of atoms
\nAgain, atomic mass of Fe = 56 u(Given)
\nThen, gram atomic mass of Fe = 56 g
\nNow, 56 g of Fe contains = 6.022 \u00d7 1023 <\/sup>number of atoms
\nThus, 100 g of Fe contains = \\(\\frac{6.022 \\times 10^{23}}{56} \\times 100\\)
\n= 1.0753 \u00d7 1024<\/sup> number of atoms
\nTherefore, 100 grams of sodium contain more number of atoms than 100 grams of iron.<\/p>\n

Class 9 Science Chapter 3 Atoms and Molecules Textbook Questions and Answers<\/span><\/h3>\n

Question 1.
\nA 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
\nAnswer:
\nMass of boron = 0.096 g (Given)
\nMass of oxygen = 0.144 g (Given)
\nMass of sample = 0.24 g (Given)
\nThus, percentage or boron by weight in the compound = \\(\\frac{0.096}{0.25}\\) \u00d7 100%
\n= 40%
\nAnd, percentage of oxygen by weight in the compound = \\(\\frac{0.144}{0.25}\\) \u00d7 100%
\n= 60%<\/p>\n

Question 2.
\nWhen 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combinations will govern your answer?
\nAnswer:
\nCarbon + Oxygen \u2192 Carbon dioxide
\n3 g of carbon reacts with 8 g of oxygen to produce 11 g of carbon dioxide.<\/p>\n

If 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left unreactive.<\/p>\n

In this case also, only 11 g of carbon dioxide will be formed.
\nThe above answer is governed by the law of constant proportions.<\/p>\n

Question 3.
\nWhat are polyatomic ions? Give examples?
\nAnswer:
\nA polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, ammonium ion (NH4<\/sub>+<\/sup>), hydroxide ion (OH–<\/sup>), carbonate ion (CO3<\/sub>2-<\/sup>), sulphate ion (SO4<\/sub>2-<\/sup>).<\/p>\n

\"NCERT<\/p>\n

Question 4.
\nWrite the chemical formulae of the following:
\n(a) Magnesium chloride
\n(b) Calcium oxide
\n(c) Copper nitrate
\n(d) Aluminium chloride
\n(e) Calcium carbonate
\nAnswer:
\n(a) Magnesium chloride \u2192 MgCl2<\/sub>
\n(b) Calcium oxide \u2192 CaO
\n(c) Copper nitrate \u2192 Cu (NO3<\/sub>)2<\/sub>
\n(d) Aluminium chloride \u2192 AlCl3<\/sub>
\n(e) Calcium carbonate \u2192 CaCO3<\/sub><\/p>\n

Question 5.
\nGive the names of the elements present in the following compounds:
\n(a) Quick lime
\n(b) Hydrogen bromide
\n(c) Baking powder
\n(d) Potassium sulphate
\nAnswer:<\/p>\n\n\n\n\n\n\n\n
Compound<\/td>\nChemical formula<\/td>\nElements present<\/td>\n<\/tr>\n
Quick lime<\/td>\nCaO<\/td>\nCalcium, Oxygen<\/td>\n<\/tr>\n
Hydrogen bromide<\/td>\nHBr<\/td>\nHydrogen bromide<\/td>\n<\/tr>\n
Baking powder<\/td>\nNaHCO3<\/sub><\/td>\nSodium, Hydrogen, Carbon, Oxygen<\/td>\n<\/tr>\n
Potassium sulphate<\/td>\nK2<\/sub>SO4<\/sub><\/td>\nPotassium, Sulphur, Oxygen<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Question 6.
\nCalculate the molar mass of the following substances:
\n(a) Ethyne, C2<\/sub>H2<\/sub>
\n(b) Sulphur molecule, S8<\/sub>
\n(c) Phosphorus molecule, P4<\/sub> (atomic mass of phosphorus = 31)
\n(d) Hydrochloric acid, HCl
\n(e) Nitric acid, HNO3<\/sub>
\nAnswer:
\n(a) Molar mass of ethyne, C2<\/sub>H2<\/sub> = 2 \u00d7 12 + 2 \u00d7 1 = 26g
\n(b) Molar mass of sulphur molecule, S8<\/sub> = 8 \u00d7 32 = 256 g
\n(c) Molar mass of phosphorusjnolecule, P4<\/sub> = 4 \u00d7 31 = 124 g
\n(d) Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g
\n(e) Molar mass of nitric acid, HNO3<\/sub> = 1 + 14 + 3 \u00d7 16 = 63 g<\/p>\n

Question 7.
\nWhat is the mass of
\n(a) 1 mole of nitrogen atoms?
\n(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
\n(c) 10 moles of sodium sulphite (Na2<\/sub>SO3<\/sub>)?
\nAnswer:
\n(a) The mass of 1 mole of nitrogen atoms is 14 g
\n(b) The mass of 4 moles of aluminium atoms is (4 \u00d7 27) g = 108 g
\n(c) The mass of 10 moles of sodium sulphite (Na2<\/sub>SO3<\/sub>) is
\n10 \u00d7 [2 \u00d7 23 + 32 + 3 \u00d7 16] g = 10 \u00d7 126 g = 1260 g<\/p>\n

\"NCERT<\/p>\n

Question 8.
\nConvert into mole.
\n(a) 12 g of oxygen gas
\n(b) 20 g of water
\n(c) 22 g of carbon dioxide
\nAnswer:
\n(a) 32 g of oxygen gas = 1 mole
\nThen, 12 g of oxygen gas = \\(\\frac {12}{32}\\) = 0.375 mole
\n(b) 18 g of water = 1 mole
\nThen, 20 g of water= \\(\\frac {20}{18}\\) = 1.11 moles (approx)
\n(c) 44 g of carbon dioxide = 1 mole
\nThen, 22 g of carbon dioxide = \\(\\frac {22}{44}\\) = 0.5 mole<\/p>\n

Question 9.
\nWhat is the mass of:
\n(a) 0.2 mole of oxygen atoms?
\n(b) 0.5 mole of water molecules?
\nAnswer:
\n(a) Mass of one mole of oxygen atoms = 16 g
\nThen, mass of 0.2 mole of oxygen atoms = 0.2 \u00d7 16g = 3.2g
\n(b) Mass of one mole of water molecule=18 g
\nThen, mass of 0.5 mole of water molecules = 0.5 \u00d7 18 g = 9 g<\/p>\n

Question 10.
\nCalculate the number of molecules of sulphur (S8<\/sub>) present in 16 g of solid sulphur.
\nAnswer:
\n1 mole of solid sulphur (S8<\/sub>) = 8 \u00d7 32 g = 256 g
\ni. e., 256 g of solid sulphur contains = 6.022 \u00d7 1023<\/sup> molecules
\nThen, 16 g of solid sulphur contains \\(\\frac{6.022 \\times 10^{23}}{256} \\times 16\\) molecules
\n= 3.76 \u00d7 102<\/sup>2<\/sup> molecules (approx)<\/p>\n

Question 10.
\nCalculate the number of aluminium ions present in 0.051 g of aluminium oxide.
\n(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
\nAnswer:
\n1 mole of aluminium oxide (Al2<\/sub>O3<\/sub>) = 2 \u00d7 27 + 3 \u00d7 16 = 102 g
\ni. e., 102 g of Al2<\/sub>O3<\/sub> = 6.022 \u00d7 1023<\/sup> molecules of Al2<\/sub>O3<\/sub>
\nThen, 0.051 g of Al2<\/sub>O3<\/sub> contains
\n= \\(\\frac{6.022 \\times 10^{23}}{102} \\times 0.051\\)
\n= 3.011 \u00d7 1020<\/sup> molecules of Al2<\/sub>O3<\/sub>
\nThe number of aluminium ions (Al3+<\/sup>) present in one molecule of aluminium oxide is 2.<\/p>\n

Therefore, the number of aluminium ions (Al3+<\/sup>) present in 3.011 \u00d7 1020<\/sup> molecules (0.051 g) of aluminium pxide (Al2<\/sub>O3<\/sub>) = 2 \u00d7 3.011 \u00d7 1020<\/sup>
\n= 6.022 \u00d7 1020<\/sup><\/p>\n

Class 9 Science Chapter 3 Atoms and Molecules Additional Important Questions and Answers<\/span><\/h3>\n

Multiple Choice Questions<\/span>
\nChoose the correct option:<\/span><\/p>\n

Question 1.
\nWhich of the following correctly represents 360 g of water?
\n(i) 2 moles of H2<\/sub>O
\n(ii) 20 moles of water
\n(iii) 6.022 \u00d7 1023 molecules of water
\n(iv) 1.2044 \u00d7 1025 molecules of water
\n(a) (i)
\n(b) (i) and (iv)
\n(c) (ii) and (iii)
\n(d) (ii) and (iv)
\nAnswer:
\n(d) (ii) and (iv)<\/p>\n

Question 2.
\nWhich of the following statements is not true about an atom?
\n(a) Atoms are not able to exist independently
\n(b) Atoms are the basic units from which molecules and ions are formed
\n(c) Atoms are always neutral in nature
\n(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch
\nAnswer:
\n(a) Atoms are not able to exist independently<\/p>\n

Question 3.
\nThe chemical symbol for nitrogen gas is
\n(a) Ni
\n(b) N2<\/sub>
\n(c) N+<\/sup>
\n(d) N
\nAnswer:
\n(b) N2<\/sub><\/p>\n

\"NCERT<\/p>\n

Question 4.
\nThe chemical symbol for sodium is
\n(a) So
\n(b) Sd
\n(c) NA
\n(d) Na
\nAnswer:
\n(d) Na<\/p>\n

Question 5.
\nWhich of the following would weigh the highest?
\n(a) 0.2 mole of sucrose (C12<\/sub>H22<\/sub>O11<\/sub>)
\n(b) 2 moles, of CO2<\/sub>
\n(c) 2 moles of CaCO3<\/sub>
\n(d) 10 moles of H2<\/sub>O
\nAnswer:
\n(c) 2 moles of CaCO3<\/sub><\/p>\n

Question 6.
\nWhich of the following has maximum number of atoms?
\n(a) 18g of H2<\/sub>O
\n(b) 18g of O2<\/sub>
\n(c) 18g of CO2<\/sub>
\n(d) 18g of CH4<\/sub>
\nAnswer:
\n(d) 18g of CH4<\/sub><\/p>\n

Question 7.
\nWhich of the following contains maximum number of molecules?
\n(a) 1g CO2<\/sub>
\n(b) 1g N2<\/sub>
\n(c) 1g H2<\/sub>
\n(d) 1g CH4<\/sub>
\nAnswer:
\n(c) 1g H2<\/sub><\/p>\n

Question 8.
\n3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of oxygen atoms in the solution are
\n(a) 6.68 \u00d7 1023<\/sup>
\n(b) 6.09 \u00d7 1022<\/sup>
\n(c) 6.022 \u00d7 1023<\/sup>
\n(d) 6.022 \u00d7 1021<\/sup>
\nAnswer:
\n(a) 6.68 \u00d7 1023<\/sup><\/p>\n

Question 9.
\nA change in the physical state can be brought about
\n(a) only when energy is given to the system
\n(b) only when energy is taken out from the system
\n(c) when energy is either given to, or taken out from the system
\n(d) without any energy change
\nAnswer:
\n(c) when energy is either given to, or taken out from the system<\/p>\n

\"NCERT<\/p>\n

Very Short Answer Type Questions<\/span><\/p>\n

Question 1.
\nDefine law of conservation of mass.
\nAnswer:
\nIn a chemical reaction mass can neither be created nor destroyed.
\nE.g., 2Na + Cl2<\/sub> \u2192 2NaCl
\n2 \u00d7 23 + 2 \u00d7 35.5 \u2192 2(23 + 35.5)<\/p>\n

Question 2.
\nExplain law of constant proportion.
\nAnswer:
\nIn a chemical substance the elements are always present in definite proportions by mass.
\nE.g., In water, the ratio of the mass of hydrogen to the mass of oxygen H : O is always 1 : 8<\/p>\n

Question 3.
\nWho coined the term atom?
\nAnswer:
\nJohn Dalton coined the term atom.<\/p>\n

Question 4.
\nDefine atom.
\nAnswer:
\nThe smallest particle of matter, which can take part in a chemical reaction is called atom.<\/p>\n

Question 5.
\nDefine molecule.
\nAnswer:
\nThe smallest particle of an element or compound which can exist independently is called molecule.<\/p>\n

Question 6.
\nDefine atomicity.
\nAnswer:
\nThe number of atoms constituting a molecule is known as its atomicity.<\/p>\n

Question 7.
\nWhat is atomic mass unit?
\nAnswer:
\nThe sum of the atomic masses of all die atoms in a molecule of the substance is expressed in atomic mass unit.
\nE.g., H2<\/sub>O = (1 \u00d7 2) + 16 = 18 amu<\/p>\n

\"NCERT<\/p>\n

Question 8.
\nHow do atoms exist?
\nAnswer:
\nAtoms exist in the form of atom, molecule or ions.<\/p>\n

Question 9.
\nGive the atomicity of phosphorous and nitrogen.
\nAnswer:
\nThe atomicity of phosphorus is P4<\/sub> i.e., 4.
\nThe atomicity of nitrogen is N2<\/sub> i.e., 2.<\/p>\n

Question 10.
\nWhat is an ion?
\nAnswer:
\nCharged atom is called as an ion. The ion can be positively charged called cation or negatively charged called anion.<\/p>\n

Question 11.
\nGive one example of cation and anion.
\nAnswer:
\nCation \u21d2 Na+<\/sup>
\nAnion \u21d2 Cl–<\/sup><\/p>\n

Question 12.
\nGive one difference between cation and anion.
\nAnswer:
\nCations are positively charged ion. Anions are negatively charged ion.<\/p>\n

Question 13.
\nGive the chemical formula for ammonium sulphate.
\nAnswer:
\nAmmonium sulphate NH4<\/sub>+<\/sup> SO4<\/sub>2-<\/sup>
\nChemical formula \u2192 (NH4<\/sub>)2<\/sub>SO4<\/sub>.<\/p>\n

Question 14.
\nWhat is Avogadro’s constant?
\nAnswer:
\nThe Avogadro’s constant (6.022 \u00d7 1023<\/sup>) is defined as the number of atoms that are present inexactly 12 g of carbon-12.<\/p>\n

Question 15.
\nFind the molecular mass of H2<\/sub>O.
\nAnswer:
\nMolecular mass of H2<\/sub>O=(2 \u00d7 1) + (16) = 2 + 16 = 18 u<\/p>\n

\"NCERT<\/p>\n

Short Answer Type Questions<\/span><\/p>\n

Question 1.
\nGive the unit to measure size of atom and give size of hydrogen atom.
\nAnswer:
\nThe unit to measure size of atom is nanometer, size of hydrogen atom is 10-10<\/sup> m.<\/p>\n

Question 2.
\nWhat is IUPAC, give its one function?
\nAnswer:
\nIUPAC stands for International Union of Pure and Applied Chemistry. It approves the names of elements.<\/p>\n

Question 3.
\nGive the Latin name for sodium, potassium, gold and mercury.
\nAnswer:
\nSodium \u2192 Natrium, Gold \u2192 Aurum Potassium \u2192 Kalium, Mercury \u2192 Hydrargyrum<\/p>\n

Question 4.
\nWhat is the ratio by mass of combining elements in H2<\/sub>O,CO2<\/sub> and NH3<\/sub>?
\nAnswer:
\nH2<\/sub>O ratio by mass of combining elements 2 : 16 \u2192 1 : 8 (H : O)
\nCO2<\/sub> ratio by mass of combining elements 12 : 32 \u2192 3 : 8 (C : O)
\nNH3<\/sub> ratio by mass of combining elements 14 : 3 \u2192 14 : 3 (N : H)<\/p>\n

Question 5.
\nDefine valency and give the valency for the following elements:
\nMagnesium, Aluminium, Chlorine and Copper.
\nAnswer:
\nValency: The combining capacity of an element is called its valency.
\nValency of the following elements:
\nMagnesium \u2014 2
\nAluminium \u2014 3
\nChlorine \u2014 1
\nCopper \u2014 2<\/p>\n

Question 6.
\nWhat is polyatomic ion? Give one example.
\nAnswer:
\nA group of atoms carrying a charge is known as a polyatomic ion.
\nE.g., Ammonium \u2014 NH4<\/sub>–<\/sup>
\nNitrate \u2014 NO3<\/sub>–<\/sup><\/p>\n

\"NCERT<\/p>\n

Question 7.
\nWrite down the formula for:
\nCopper nitrate, calcium sulphate and aluminium hydroxide.
\nAnswer:
\nChemical formula:
\nCopper nitrate \u2192 Cu(NO3<\/sub>)
\nCalcium sulphate \u2192 CaSO4<\/sub>
\nAluminium hydroxide \u2192 Al(OH)3<\/sub><\/p>\n

Question 8.
\nWhat is formula unit mass? How is it different from molecular mass?
\nAnswer:
\nThe formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. The constituent particles of formula unit mass are ions and the constituent particles of molecular mass are atoms.<\/p>\n

Question 9.
\nFind the number of moles in the following:
\n(i) 50g of H2<\/sub>O
\n(ii) 7g of Na
\nAnswer:
\nNumber of moles in
\n(i) Molar mass of H2<\/sub>O = 18g
\nGiven mass of H2<\/sub>O = 50 g
\n\u2234 No. of moles in 50g of H2<\/sub>O
\n= \\(\\frac {50}{18}\\) = 2.78 moles.<\/p>\n

(ii) Molar mass of Na = 23g
\nGiven mass of Na = 7g
\n\u2234 No. of moles in 7g of Na = \\(\\frac {7}{23}\\) = 0.304 moles.<\/p>\n

Question 10.
\nFind the number of atoms in the following:
\nAnswer:
\n(i) 0.5 mole of C atom
\n(ii) 2 mole of N atom
\nAnswer:
\n(i) 0.5 mole of C atom:
\nNumber of atoms in 1 mole of C atom
\n= 6.022 \u00d7 1023<\/sup> atoms
\nNumber of atoms in 0.5 mole of C atom
\n= 6.022 \u00d7 1023<\/sup> \u00d7 0.5
\n= 3.011 \u00d7 1023<\/sup> atoms<\/p>\n

(ii) 2 mole of N atom:
\nNumber of atoms in 1 mole of N atom = 6.022 \u00d7 1023<\/sup> atoms
\n\u2234 Number of atoms in 2 mole of N atom
\n= 6.022 \u00d7 2 \u00d7 1023<\/sup>
\n= 1.2044 \u00d7 1024<\/sup> atoms<\/p>\n

\"NCERT<\/p>\n

Question 11.
\nFind the mass of the following:
\n(i) 6.022 \u00d7 1023<\/sup> number of O2<\/sub> molecules
\n(ii) 1.5 mole of CO2<\/sub> molecule
\nAnswer:
\n(i) 6.022 \u00d7 1023<\/sup> number of O2<\/sub> molecules:
\nMass of 1 mole of O2<\/sub> molecule = 6.022 \u00d7 1023<\/sup> molecules = 32 g<\/p>\n

(ii) 1.5 mole of CO2<\/sub> molecule:
\nMass of 1 mole of CO2<\/sub> molecule = 6.022 \u00d7 1023<\/sup> molecules = 44 g
\nMass of 1.5 mole CO2<\/sub> molecule = 44 \u00d7 1.5 = 66 g<\/p>\n

Question 12.
\nWhat are the rules for writing the symbol of an element?
\nAnswer:
\nIUPAC \u2192 International Union of Pure and Applied Chemistry approves name of elements.<\/p>\n

Symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (upper case) and the second letter as a small letter (lower case).
\ne.g., Hydrogen \u2192 H
\nHelium \u2192 He<\/p>\n

Some symbols are taken from the names of elements in Latin, German or Greek.
\ne.g., Symbol of iron is Fe, its Latin name is Ferrum.
\nSymbol of sodium is Na, its Latin name is Natrium.<\/p>\n

Question 13.
\nExplain relative atomic mass and relative molecular mass.
\nAnswer:
\nRelative atomic mass: It can be defined as the number of times one atom of given element is heavier than \\(\\frac {1}{2}\\) th of the mass of an atom of carbon-12.<\/p>\n

Relative Molecular Mass: It is defined as the number of times one molecule of a substance or given element is heavier than vyth of the mass of one atom of carbon-12.<\/p>\n

Question 14.
\nThe formula of carbon-dioxide is CO2<\/sub>. What information do you get from this formula?
\nAnswer:<\/p>\n