Page 1 :
(iv) What is the ratio of number of atoms for water?, Answers, (1) It is the umber of atoms present in one molecule of a substance., (i) Ozone (O,), (im) (a) 8, (b) 4, Ans, (i) The chemical formula of water is H,O. There are two atoms of hydrogen and one ato, oxygen. So, the ratio of H:O is 2:1., An, 3. Molecules are the essential building blocks of life. Without water molecules, life on Earth w, not exist. All living organisms need protein molecules for structure and function. Therefor, understanding of how atoms combine to form molecules is key to understanding the origins of, An atom or a molecule can lose or gain electron(s) to form an ion. When an atom/molecule, negatively charged electron(s), a negative ion is formed. When an atom/molecule loses negativ A, charged electron(s), a positive ion is formed. A molecule is a neutral particle, composed of a, number of atoms bonded together. The particle of the substance is the molecule, rather than, atoms that make up the molecule. By contrast, ionic compounds are made up of an indetermin, number of ions, in a fixed ratio. The particle of the ionic substance remains the ion., Source: Royal Society of Chemistry-'How to teach atoms, molecules and ions' by Catherine Smith, 20 June 2019, Based on above information, answer the following questions., (i) What is an ion? Give one example., (ii) Give one word for the following:, (a) Negatively charged ion, (iii) Write the anion and cation present in KCI., (b) Positively charged ion, (iv) What are polyatomic ions? Give an example., Answers, (i) The negatively and positively charged particles are called ions., For example: Cl", Br", SO, PO, H", Pb", etc. (any one), (ii) (a) Anion, (b) Cation, (iii) Anion is Cl and cation is K*., (iv) A group of atoms carrying a charge is known as a polyatomic ion. For example, phosphat, ion PO., SHORT ANSWER QUESTIONS-I, [2 marks], 1. State the number of atoms present in each of the following chemical species, [NCERT Exemplar, (a) CO, (b) PO, (b) 5, (c) P,O,, (d) CO, (c) 7, (d) 2, (a) 4, 2. Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12, Molar atomic mass of magnesium is 24g mol., Ans., [NCERT Exemplar, 12, = 0.5 mol, 24, Ans. Number of moles =, 12 Science-IX: Term-2

Page 2 :
1. Express cach of the following in kilograms, (a) 5.84 x 10, (b) 58.34 g, (b) 5.834 x 10 kg, Calculate the mass in grams of a single atom of oxygen., mg, INCERT Exemplar], (a) 5.84 x 10, kg, (c) 0.584g, (d) 5.873 x 10"g, (d) 5.873 x 10 kg, Ans., () 5.84 x 10 kg, (Atomic maSs of oxygen = 16)., Ans 6.022 x 10" atoms of oxygen = Molar mass of oxygen =16 g, one atom, Mass of 1 atom of oxygen, Earth wou, herefore, rigins of li, lecule ga, s negative, sed of a s, 16, 6.022x10 8, %D, = 2.66 x 1023, g, 5. What is the percentage of sulphur in sulphuric acid?, [At. mass: H = 1; S = 32; 0 = 16], Ans. Molecular mass of H,SO, = 2H + 1S+40, = (2 x 1) + (1 x 32) + (4 x 16), = 2 + 32 + 64 = 98 u, er than ti, etermina, 32, % of S in H,SO,, х 100, %3D, 98, = 32.65%, e Smith o, 6. Calculate the formula mass of sodium carbonate (Na, CO,.10H O)., Ans. Formula mass of sodium carbonate, = (2 x atomic mass of Na) + (1 x atomic mass of C) + (3 x atomic mass of O), + 10 [(2 x atomic mass of H) + (1 × atomic mass of O)], = 2 x 23 + 1 x 12 + 3 x 16 + 10 [(2 x 1) + (1 x 16)], = 46 + 12 + 48 + 180 = 286 u, 7. A sample of ammonia weighs 3.00 g. What mass of sulphur trioxide contains the same number, of molecules as are in 3.00 g ammonia?, 3.00, mol, Ans. Number of moles of ammonia in 3.00 g =, 17, = 0.1764 mol, Molecular mass of SO, = 1 x 32 u + 3 x 16 u = 80 u, I mole of SO, weighs 80 g, 0.1764 moles weigh = 80 x 0.1764 g, = 14.11 g, %3D, osphate, %3D, 8. Write the cations and anions present (if any) in the following compounds:, (i) CH,COONA, (iii) H2, (ii) NaCl, INCERT Exemplar, (iv) NH,NO,, ks], Ans., Anions, Cations, (1) CH,COO, Na, plar), (ii) C, Na, (iii) H-It is a covalent compound, (iv) NO,, 3. You are provided with a fine white coloured powder which is either sugar or salt. How would, you identify it without tasting?, Ans. On heating the powder, it will char if it is a sugar., Alternatively, the powder may be dissolved in water and checked for its conduction of electricity., If it conducts. it is a salt., NH, 12 g, INCERT Exemplar [HOTS], plar, Atoms and Molecules | 13

Page 3 :
13 man, SHORT ANSWER QUESTIONS-II, 1. Which of the following symbols of elements are incorrect? Give their correct symk, (ii) Carbon, 5. In a, What, CO, (i) Cobalt, Ans. Since, He, AL, (ir) Helium, INCERT Ex, 3g ca, (iii) Aluminium, () Sodium, So, (1) Incorrect, the correct symbol of cobalt is Co., (1) Incorrect, the correct symbol of carbon is C., (i) Incorrect, the correct symbol of aluminium is Al., Ans., Law, (iv) Correct (He), pre, 6. Cla, (7) Incorrect, the correct symbol of sodium is Na., 2., () Differentiate between the actual mass of a molecule and gram molecular mass,, (ii) Calculate the mass of one atom of hydrogen atom., (ix, () Actual mass of a molecule is obtained by dividing the molar mass by Avogadro's nus, whereas gram molecular mass represents the molecular mass expressed in grams,, is the mass of 1 mole of molecules, i.e., Avogadro's number of molecules., (in) 1 mole of hydrogen atom = 1g, Ans., Ans., (, 6.022 x 10 atoms of hydrogen weigh 1g, %D, or, 1, Mass of one atom =, 6.022x103 5, = 1.66058 x 10-24, 7., 3. How many atoms would be present in a black dot marked on the paper with graphite penci, a full stop at the end of a sentence., Ans., [Given mass of a dot = 10ls gl, Ans. I mole of carbon atoms weigh 12 g, !!, Also, I mole of carbon atoms, = 6.022 x 10, atoms, Thus, 12 g of carbon atoms has 6.022 x 10 atoms., 6.022X1023, 10-18, g of carbon will have, X10-1s carbon atoms, 12, 8., = 5.02 x 10' carbon atoms., Ans., 4. How many times heavier is one atom of carbon than one atom of oxygen?, Ans. Mass of 6.022 x 10 atoms of carbon =, 12 g, 12, Mass of 1 atom of carbon =, g., 6.022x1023, Mass of 6.022 x 10 atoms of oxygen, = 16 g, 16, Ans, Mass of 1 atom of oxygen =, 6.022x1023 5, On dividing (1) by (2), we get, Mass of carbon atom, 12, Mass of oxygen atom, 16, 3., x mass of oxygen atom, 4, Mass of carbon atom =, 3, times that of oxygen atom., So, mass of carbon atom is, 4, 14, Science-IX: Term-2

Page 4 :
ks, 5. In a compound, carbon and oxygen react in a ratio of 3:8 by mass to form carbon dioxide., What mass of oxygen is required to react completely with 9 g carbon: State the associated law., Ans. Since carbon and oxygen combine in the ratio of 3:8 by mass, it means, 3g carbon reacts with 8 g of oxygen atoms, 8., of oxygen atoms, 38, . lg carbon reacts with, 8x9, 38 Le., 24 g of oxygen atoms, . 9g carbon reacts with, Law of constant proportion states that "in a pure chemical compound, elements are always, present in a definite proportion by mass.", 6. Classify each of the following on the basis of their atomicity., (i) F2, (ii) NO,, (iii) Co, (in) C,H,, (viii) O,, (2) CO, (vi) H,O,, (vii) P,O10, (ix) HCI, (x) CH,, (xi) He, (xii) Ag, [NCERT Exemplar], Ans., (i) 2, (ii) 3, (iii) 4, (rv) 8, (7) 2, (vi) 4, (vii) 14, (vii) 3, (ix) 2, (x) 5, (xi) 1 (Noble gases do not combine and exist as monoatomic gases), (xii) Polyatomic: It is difficult to talk about the atomicity of metals as any measurable quantity, will contain millions of atoms bound by metallic bond., 7. Calculate the molecular mass of the following:, (i) H,CO,, (ii) C,H,OH, (iii) MgSO,, Ans., (i) Molecular mass of H,CO, = 2 x1+ 1 x 12 +3 x 16, = 2 + 12 + 48 62 u, (ii) Molecular mass of C,H,OH = 2 x 12 + 5 x1+1 x 16 + 1, = 24 + 5+ 16 +1 = 46 u, (in) Molecular mass of MgSO, = 1x 24 +1 x 32 +4 x16, = 24 + 32 + 64 = 120 u, INCERT Exemplar], 8. What are ionic and molecular compounds? Give examples., Ans. Atoms of different elements join together in definite proportions to form molecules of compounds., For example, water, ammonia, carbon dioxide. Compounds composed of metals and non-metals, contain charged species. The charged species are known as ions. An ion is a charged particle, and can be negatively or positively charged. A negatively charged ion is called an anion and the, positively charged ion is called cation. For example, sodium chloride, calcium oxide., 9. How many (i) molecules (ii) hydrogen atoms (iii) oxygen atoms are there in 0.5 mol of water?, (i) 1 mol of water contains 6.022 x 10 molecules, Ans., 6.022x1023, molecules, 0.5 mol of water contains, = 3.011 x 10 molecules, (i) 1 molecule of water contains 2 atoms of hydrogen, 1 mol of water contains 2 x 6.022 x 10 atoms of hydrogen, 2x6.022x10, atoms of hydrogen, 0.5 mol of water contains, = 6.022 x 10 atoms of hydrogen, Atoms and Molecules | 15

Page 5 :
(m) I molecule of water contains I atom of oxygen, I mol of water contains 6.022 x 10 atoms of oxygen, 6.022x 10, 2., atoms of oxygen, 0.5 mol of water contains, = 3.011 x 1023, atoms of oxygen, 10. Calculate the number of moles present in:, 3.011 x 10 number of oxygen atoms., () 60 g of calcium, [Given that atomic mass of Ca =40 u, Avogadro No. 6.022 x 101, () I mole of oxygen contains 6.022x10, 6.022 x 10, atoms, 13. F, Ans., atoms of oxygen =1 mol, 1 atom of oxygen =, mol, 6.022x105, Ans.., 1x 3.011x10, 23, 6.022x10, 3.011 x 10 atoms of oxygen =, mol, = 0.5 mol, (u) Atomic mass of Ca = 40 u, 40g of calcium = 1 mol, 14., 60, mol = 1.5 mol, 40, 60g of calcium =, 1. Calculate the mass per cent of each element of sodium chloride in one mole of it., Ans., Ans. Molecular mass of NaCl = (1 x 23 +1x 35.5) u = 58.5 u, Atomic mass of sodium = 23 u, 15., Atomic mass of Na, Mass percent of Na=, x100, Molecular mass of NaCl, Ans., 23, -x100 39.32%, 58.5, Mass % of Na 39.32 %, Atomic mass of chlorine 35.5 u, Atomic mass of Cl, Mass % of CI =, x100, Molecular mass of NaCl, 16., 35.5, x100 = 60.68 %, 58.5, 12. Calculate the number of particles in each of the following:, Ans., (i) 46 g of Na atom, (i) 8 g of O, molecules, (i) 0.1 moles of carbon atom, Ans., (1) No. of moles of sodium =, 46, = 2 moles, 23, We know that one mole of sodium contains 6.022x10 atoms., :-, 2 moles of sodium contain= 2 x 6.022 x 10 atoms, = 1.204 x10" atoms, 16 | Science-IX: Term-2