(M) 9634239157

Sanket Shah

SHAH EDUCATION CENTRE(Tuition for Classes) House No. 50, Sector 2 Vibhav Nagar, Agra-1 Atoms and Molecules - Class IX

Subtitle All matter is made up of small particles called atoms and molecules. The properties of matter depend on the properties of atoms and molecules from which it is made. Laws of chemical combination Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination. These laws were established after much experimentation by Lavoisier and Joseph L. Proust. (i.) Law of conservation of mass It states that mass can neither be created nor destroyed in a chemical reaction. - It means that in a chemical reaction, the total mass of products is equal to the total mass of reactants. There is no change in mass during a chemical reaction. - When calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide. It has been

found by experiments that if 100 g of CaCO3 is decomposed completely then 56 g of CaO and 44 g of CO2 are formed. CaCO3 CaO + CO2 Since the total mass of products (56 + 44 = 100 g) is equal to the total mass of reactants (100 g), there is no change of mass during the reaction. - Law of conservation of mass holds true only if the reaction is carried out in a closed container. Then no matter can leave or enter the container. (ii.) Law of constant proportions The law was stated by Proust as In a chemical substance the elements are always present in definite proportions by mass. - It is also known as law of definite proportions. - It means that whatever be the source from which it is obtained or the method by which it is prepared, a pure chemical compound is always made up of the same elements in the same proportion by mass. - In water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained. - In ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or source from which it is obtained. Daltons atomic theory John Dalton proposed the basic theory about the nature of matter. - The postulates of his theory are as follows: (i.) All matter, whether an element, a compound, or a mixture is composed of very tiny particles called atoms.

(ii.) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (iii.) Atoms of a given element are identical in mass and chemical properties. (iv.) Atoms of different elements have different masses and chemical properties. (v.) Atoms combine in the ratio of small whole numbers to form compounds. (vi.) The relative number and kinds of atoms are constant in a given compound. - Daltons atomic theory was based on the laws of chemical combination. - The postulate of Daltons atomic theory that atoms can neither be created nor destroyed was the result of law of conservation of mass. - The postulate that the elements consist of atoms having fixed mass, and that the number and kind of atoms in a given compound is fixed came from the law of constant proportions. - His theory also provided an explanation for the laws of chemical combination. - According to his theory atoms can neither be created nor destroyed. Since this is true, therefore, the number of various types of atoms in the products of a chemical reaction is the same as the number of all those atoms in the reactants. The same number of various atoms in products and reactants will have same mass. So, the total mass of products is equal to the total mass of reactants. And this is law of conservation of mass. - According to his theory, the number and kinds of atoms are constant in a given compound. Since, this is true,

therefore, a compound will always have the same elements combined together in the same proportion by mass. And this is law of constant proportions. - Drawbacks of Daltons atomic theory It is now known that some statements of his theory are not correct. These are: (i.) Atoms were indivisible according to Dalton. We now know that under special circumstances, atoms can be further divided into still smaller particles called electrons, protons and neutrons. (ii.) Daltons atomic theory says that all the atoms of an element have exactly the same mass. It is, however, now known that atoms of the same element can have slightly different masses as in isotopes. (iii.) His theory says that atoms of different elements have different masses. It is, however, now known that even atoms of different elements can have the same mass as in isobars. Atom An atom is the smallest particle of an element that can take part in a chemical reaction. - Atoms are the building blocks of all the matter around us. - Atoms are so small in size, that they cannot be seen even under the most powerful light microscope. - The size of an atom is indicated by its radius, which is called atomic radius. - Atomic radius is measured in nanometers (nm). 1 nm = 10-9 m Symbols of elements Dalton was the first scientist to use the symbols to represent elements in a short way.

- His symbol for an element represented the element as well as one atom of that element. - J. J. Berzelius suggested that the symbols of elements be made from one or two letters of the name of the element. - Now-a-days, IUPAC i.e. International Union of Pure and Applied Chemistry approves names of elements. - The symbol of an element is the first letter or the first letter and another letter of the English name or Latin name of the element. - Importance of symbol of an element: (i.) It represents name of the element. (ii.) It represents one atom of the element. (iii.) It represents one mole or 6.022 x 1023 atoms of the elements. (iv.) It represents a definite mass (atomic mass in grams) of the element. - Examples are chlorine Cl, Zinc Zn, Gold Au, Sodium Na, Potassium K, Oxygen O, Nitrogen N, etc. - The first letter of a symbol is always written as a capital letter and the second letter as a small letter. Atomic mass According to Daltons atomic theory, each element has a characteristic atomic mass. - While searching for various atomic mass units, scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit. This was considered because: (i.) Oxygen reacted with a large number of elements and

formed compounds. (ii.) This atomic mass unit gave masses of most of the elements as whole numbers. - However, in 1961, for a universally accepted atomic mass unit, carbon 12 isotope was chosen as the standard reference for measuring atomic masses. - One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon 12. - Atomic mass unit is written in short as u which means unified mass. - Since determining the mass of an individual atom is a relatively difficult task, relative atomic masses of all the elements have been found with respect to an atom of carbon 12. - The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom. Existence of atoms The atoms of only a few elements called noble gases are chemically unreactive and exist in the free state as single atoms. - Atoms of most elements are chemically very reactive and do not exist independently. - Atoms form molecules and ions, which aggregate in large numbers to form different types of matter. Molecule A molecule is the smallest particle of an element or a compound that is capable of an independent existence and show all the properties of that substance. - In general, a molecule is a group of two or more atoms that are chemically bonded together.

- Atoms of the same element or of different elements can join together to form molecules. Accordingly there are two types of molecules: (i.) Molecules of elements The molecule of an element contains two or more similar atoms chemically combined together. - For example, hydrogen gas consists of H2 molecules, nitrogen gas consists of N2 molecules, sulphur consists of S8 molecules, ozone consists of O3 molecules, phosphorus consists of P4 molecules, etc. - The number of atoms constituting a molecule is known as its atomicity. Thus the atomicity of hydrogen is 2, sulphur is 8, ozone is 3, phosphorus is 4, etc. - Molecules of metals and some other elements such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together. (ii.) Molecules of compounds The molecule of a compound contains two or more different types of atoms chemically combined together in a definite proportion. - For example, hydrogen chloride molecule contains hydrogen and chlorine atoms, water molecule consists of hydrogen and oxygen atoms, etc. Ions An ion is an electrically charged particle. - It is formed by the loss or gain of one or more valence electrons by an atom, so it contains an unequal number of electrons and protons. - There are two types of ions cations and anions. (i.) Cations are positively charged ions. - They are formed by the loss of one or more electrons

by an atom. - e.g. Sodium atom loses one electron to form a sodium ion, Na+, which is a cation. Na 1e- Na+ Sodium atom Electron Sodium ion - Since a cation is formed by the removal of electrons from an atom, therefore a cation contains less electrons than a normal atom. - The atomic number of the atom does not change upon the formation of a cation. This is because the number of protons in a cation is the same as that in the parent atom. - A cation is smaller in size than its parent atom. - A cation is stable as compared to its parent atom. - The number of electrons in a cation is less than the number of protons. - A cation is unreactive as it usually contains 8 electrons in its outermost shell. - The ions of all the metal elements are cations, e.g. Na+, K+, Ca+2, Mg+2, Al+3, etc. H+ and NH4+ are the only cations formed from non-metals. (ii.) Anions are negatively charged ions. - They are formed by the gain of one or more electrons by an atom. - e.g. Chlorine atom gains one electron to form chloride ion, Cl-, which is an anion. Cl + 1e- ClChlorine atom Electron Chloride ion

- Since an anion is formed by the gain of electrons by an atom, therefore, an anion contains more electrons than a normal atom. - The atomic number of the atom does not change upon the formation of an anion. This is because the number of protons in an anion is the same as that in the parent atom. - An anion is bigger in size than its parent atom. - An anion is stable as compared to its parent atom. - The number of electrons in an anion is more than the number of protons. - An anion is unreactive as it usually contains 8 electrons in its outermost shell. - The ions of all non-metals are anions (except H+ and NH4+), e.g. Cl-, O-2, S-2, N-3, etc. Simple ions and compound ions Those ions which are formed from single atoms are called simple ions, e.g. sodium ion Na+, magnesium ion Mg+2, oxide ion O-2, etc. - Simple ions are also called monoatomic ions. - Those ions which are formed from group of atoms joined together are called compound ions, e.g. ammonium ion NH4+, hydroxide ion OH-, carbonate ion CO3-2, etc. - Compound ions are also called polyatomic ions. Ionic compounds The compounds which are made up of ions are called ionic compounds. - In an ionic compound, the cations and anions are held together by strong electrostatic forces of attraction.

- e.g. Sodium chloride is an ionic compound which is made up of Na+ and Cl- ions. Chemical formula The chemical formula of a compound represents the composition of a molecule of the compound in terms of the symbols of the elements present in it. - The formula of a compound tells us the kind of atoms as well as the number of atoms of various elements present in one molecule of the compound. - In case of molecular compounds, the chemical formula represents the composition of molecule which makes up the compound. - In case of ionic compounds, the chemical formula represents the simplest ratio of ions present in the compound. - A compound made up of only two elements is called a binary compound, e.g., MgO, CaCl2, etc. Writing chemical formulae The combining capacity of an element is known as its valency. - Valency can be used to find out how the atom of an element will combine with atom(s) of another element to form a chemical compound. - While writing a chemical formula the symbols of the element which form the compound are written. Below the symbol of each element, its valency is written. Finally the valencies of the atoms are crossed over. - e.g. H +1 S -2

H2S1 or H2S - It is important that the valencies or charges on the ion must balance. - In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. e.g. NH4 +1 (NH4)2SO4 - When a compound consists of a metal and non-metal, the name or symbol of the metal is written first, whereas the non-metal is written later. Also, the metal is named as such but the name of non-metal element is changed to have the ending ide, like oxide, chloride, sulphide, etc. - In the formula, the charges on the ions are not indicated. Molecular mass The relative molecular mass of a substance is the average mass of its molecule as compared with the mass of a carbon-12 atom taken as 12 units. - The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. Formula unit mass An ionic compound is made up of an extremely large number of cations and anions. e.g. Sodium chloride is made up of a large (but equal) number of Na+ and Cl- ions so the actual formula of SO4 -2

sodium chloride should be (Na+Cl-)n but for simplicity we represent it by NaCl. So NaCl is the simplest formula of sodium chloride and not its actual formula. - The simplest combination of ions that produces an electrically neutral unit, is called a formula unit of an ionic compound. - It is the smallest unit of that compound and can be considered equivalent to a molecule of a compound. - The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound. - e.g. formula unit mass of NaCl is 23 x 1 + 35.5 x 1 = 23 + 35.5 = 58.5u. Mole concept The word mole was introduced by Wilhelm Ostwald. In Latin, mole means heap or collection or pile. - A mole is defined as the amount of a substance that contains as many particles (atoms, molecules or ions) of the substance as there are atoms in exactly 12 g of carbon-12. - The number of particles present in a mole of any substance is fixed, with a value of 6.022 x 1023. This number is called Avogadro number or Avogadro constant, named in honour of the Italian scientist Amedeo Avogadro. It is represented by N0. - In other words, a mole of a substance is that amount of the substance that contains 6.022 x 1023 particles of the substance. - Mole is the SI unit of amount of substance. - The atomic mass of an element expressed in grams is called gram atomic mass of the element.

- The molecular mass of a substance expressed in grams is called gram molecular mass of the substance. - Gram atomic mass or gram molecular mass also represents molar mass of the substance. - The mass of 1 mole of a substance is called molar mass of the substance. 1 mole = 6.022 x 1023 particles = Gram atomic mass (or gram molecular mass) Some important formulae (i.) n = m M (ii.) n = N N0 (iii.) m = M x n (iv.) m = M x N N0 (v.) N = n x N0 (vi.) N = m x N0 M n = number of moles m = given mass M = molar mass N = given number of particles N0 = Avogadro number

QuestionsQuestion 1Question: In water, hydrogen and oxygen are present in the ratio of ________. 1. 1:8 2. 2:12 3. 2:3 4. 1:2 Answer: 1

Question 2Question: An example of a triatomic molecule is ___________. 1. Ozone 2. Nitrogen 3. Carbon monoxide 4. Hydrogen Answer: 1

Question 3Question: The quantity of matter present in an object is called its _________. 1. Mass 2. Volume 3. Density 4. Vapour pressure Answer: 1

Question 4Question: Indivisibility of an atom was proposed by ___________.

1. Dalton 2. Rutherford 3. Thomson 4. Bohr Answer: 1

Question 5Question: The value of Avogadro constant is ___________. 1. 6.022 x 1024 2. 6.022 x 1022 3. 60.22 x 1023 4. 6.022 x 1023 Answer: 4

Question 6Question: All samples of carbon dioxide contain carbon and oxygen in the mass ratio of 3:8. This is in agreement with the Law of ___________. 1. Conservation of Mass 2. Constant Proportion 3. Multiple Proportion 4. Reciprocal Proportion Answer: 2

Question 7Question: The atomic mass of sodium is 23. The number of moles in 46g of sodium is ________. 1. 2 2. 1 3. 4 4. 0

Answer: 1

Question 8Question: The molecular formula of potassium nitrate is ________. 1. KNO 2. KNO3 3. KNO2 4. KON Answer: 2

Question 9Question: Kalium is the Latin name of ________. 1. Potassium 2. Krypton 3. Calcium 4. Phosphorus Answer: 1

Question 10Question: The smallest particle of a substance that is capable of independent existence is _________. 1. Atom 2. Molecule 3. Electron 4. Proton Answer: 2

Question 11Question: 1. Atomic number

2. Atomic mass 3. Atomic mass scale 4. Number of electrons Answer: 2

Question 12Question: 1. 1.25 2. 15 3. 14 4. 12 Answer: 2

Question 13Question: The molecular mass of ammonia is _________. 1. 17 grams 2. 31 grams 3. 20 grams 4. 25 grams Answer: 1

Question 14Question: Atomic mass of calcium is 40. The mass of 2.5 gram atoms of calcium is _______. 1. 40 g 2. 2.5 g 3. 100 g 4. 80 g Answer: 3

Question 15Question: The number of atoms in a molecule of the elementary substance is called ________. 1. Atomic number 2. Avogadro number 3. Atomic mass 4. Atomicity Answer: 4

Question 16Question: Avogadro number represents the number of atoms in ________. 1. 12 grams of 12C 2. 320 grams of sulphur 3. 32 grams of oxygen 4. 1 gram of 12C Answer: 1

Question 17Question: The number of moles in 5 grams of calcium is _________. 1. 0.125 mole 2. 5 mole 3. 1.25 mole 4. 12.5 moles Answer: 1

Question 18Question: 2 molecules of nitrogen are represented by ________. 1. N 2. 2N2

3. N2 4. 2N Answer: 2

Question 19Question: One mole of H2O corresponds to 1. 22.4 litre at 1 atm and 250C 2. 6.02 x 1023 atoms of hydrogen and 6.02 x 1023 atoms of oxygen 3. 18 g 4. 1 g Answer: 3

Question 20Question: The number of molecules in 4.25 g of ammonia is approximately 1. 1.0 x 1023 2. 1.5 x 1023 3. 2.0 x 1023 4. 3.5 x 1023 Answer: 2

Question 21Question: A sample contains 22 g of carbon dioxide. This is equal to 1. One molar volume of carbon dioxide 2. One mole of carbon dioxide 3. Half mole of carbon dioxide 4. Two moles of carbon dioxide Answer: 3

Question 22Question: Calculate the molar mass of HNO3. [N = 14, O = 16, H = 1] Answer: Molar mass of HNO3. H = 1 x 1 = 01 N = 14 x 1 = 14 O = 16 x 3 = 48 Total mass = 63 grams Molar mass of HNO3= 63 grams

Question 23Question: Calculate the formula mass of CaCl2. [Ca = 40, Cl = 35.5] Answer: 1(Ca) + 2(Cl) 40 + 2x(35.5) = 111 amu The formula mass of CaCl2 is 111 amu.

Question 24Question: A certain non-metal X forms two oxides I and II. The mass percentage of oxygen in oxide I (X4O6) is 43.7, which is same as that of X in oxide II. Find the formula of the second oxide. Answer: Now 43.7 parts of oxygen in I corresponds to = 6 oxygen atoms

Also 56.3 parts of X in I correspond to = 4 X atom

Now the atomic ratio X : O in the second

The formula of the second oxide is X2O5.

Question 25Question: (i) Calculate the mass of 0.2 moles of water (O=16, H=1). (ii) What is the volume of 7.1 g of chlorine (Cl=35.5) at S.T.P. Answer: (i) Gram Molecular Weight of H2O = 2 x 1 + 16 = 18 g 1 mole of water weighs 18 g

(ii) Gram Molecular Weight of Cl2 (one mole)= 35.5 x 2 = 71 g. 71 g of Cl2 at S.T.P occupies 22.4 litres

Question 26Question: The reaction between aluminium carbide and water takes place according to the following equation: Calculate the volume of CH4 released from 14.4 g of Al4C3 by excess water at S.T.P. (C = 12, Al = 27) Answer: Molecular weight of Al4C3 is (27 x 4) + (12 x 3) = 144 144 g of Al4C3 produces 3 x 22.4 litres of CH4 at S.T.P

Question 27Question: A compound of sodium, sulphur and oxygen has the following percentage composition. Na=29.11%, S=40.51%, O=30.38%. Find its empirical formula (O=16, Na=23, S=32). Answer: Empirical formula is NaSO1.5 or to its nearest whole number i.e., the formula is Na2S2O3.

Question 28Question: Solid ammonium dichromate with relative molecular mass of 252 g decomposes according to the equation.

(i) What volume of nitrogen at S.T.P will be evolved when 63 g of (NH4)2Cr2O7 is decomposed? (ii) If 63 g of (NH4)2Cr2O7 is heated above 1000C, what will be the loss of mass? (H=1, N=14, O=16, Cr=52). Answer: 252 g of (NH4)2Cr2O7 gives one mole or 22.4 litres of N2 at S.T.P as per the given equation.

(ii) At temperatures above 1000C water is in the form of steam. Products as vapours are N2 and H2O. The transformation of solids and liquids into gaseous substances results in loss of mass. Total weight of gaseous products = {(2 x 14) + 4 (2 x 1) + 16} = 28 + 72 = 100 g Heating 252 g of (NH4)2Cr2O7 causes 100 g loss of mass.

The loss of mass is 25 g

Question 29Question: How many litres of ammonia are present in 3.4 kg of it? (N = 14, H = 1) Answer: Gram molecular weight of NH3 = 14 + (1 x 3) = 17 g. 17 g of NH3 = 22.4 litres

= = 4480 litres.

Question 30Question: About 640 mL of carbon monoxide is mixed with 800 mL of oxygen and ignited in an enclosed vessel. Calculate the total volume of gases after the burning is completed. All volumes are measured at S.T.P. Answer: The chemical reaction actually taking place is:

Volume of O2 used = 320 mL. Volume of O2 left = 800 - 320 = 480 mL. Volume of CO2 formed = 640 mL. Therefore the total volume of gases after burning is 480 + 640 = 1120 mL.

Question 31Question: Calculate the number of moles of ammonium sulphate present in 15.84 kg of it. (H=1, N=14, O=16, S=32) Answer: Molecular weight of (NH4)2SO4 = (2 x 14) + (2 x 4) + 32 + (16 x 4) = 132 a.m.u. 132 g of (NH4)2SO4 = 1 mole

The number of moles of ammonium sulphate present in 15.84 kg is 120 moles.

Question 32Question: What is a symbol? What information does it convey? Answer: A short hand representation of an element is called symbol. It represents the following: Name of the element

One atom of the element One mole of atoms. It represents 6.023 x 1023 atoms of the element. A definite mass of the element

Question 33Question: What is the difference between symbol of an element and formula of an element? Answer: Symbol of an element represents the name of the element. It also represents one atom of the element. Example: H represents hydrogen and C represents carbon A formula of an element represents the number of atoms in the molecule of the compound. One molecule of hydrogen element contains two atoms of hydrogen; therefore the formula of hydrogen is H2. 2H represents two separate atoms of hydrogen, whereas H2 represents 1 molecule of hydrogen similarly the molecular formula of oxygen element and chlorine element is O2 and Cl2.

Question 34Question: Define atomic mass of an element. Answer: The atomic mass of an element is the relative mass of its atom as compared to the mass of C - 12 atom taken as 12 units.

Question 35Question: State the laws of chemical combination. Answer: Laws of chemical combinations are: i) Law of Conservation of Mass: The law states that during any physical or chemical change, the total mass of the product remains equal to the total mass of the reactants. ii) Law of Constant Composition: The law states that a chemical compound always contains same elements combined together in the same proportion by mass. iii) Law of Multiple Proportions: The law states that when two elements combine with each other to form two or more compounds, the masses of one of the elements, which combine with fixed mass of the other, bear a simple whole number ratio to one another.

Question 36Question: In an experiment it was found that litharge, red oxide of lead and lead peroxide contained 92.83%, 90.6% and 86.6% of lead respectively. Show that these figures are in agreement with the Law of Multiple Proportions. Answer: In litharge, the amount of lead = 92.83% The amount of oxygen = 100 - 92.83 = 7.17% 7.17g of O2 combines with 92.83g of lead

In red oxide of lead, the amount of Pb = 90.6% The amount of oxygen = 100 - 90.6 = 9.4 % 9.4 g of oxygen combines with 90.6g of lead

In lead peroxide, the amount of lead = 86.6% The amount of oxygen = 100 - 86.6 = 13.4% 13.4g of oxygen combines with 86.6g of lead

Thus, the different weights of lead combining with fixed weight (1g) of oxygen are in the ratio 12.947: 9.638: 6.462 = 2:1.5: 1 or 4: 3: 2 This is in line with the law of multiple proportions.

Question 37Question: What do you understand by atomicity of an element? Give an example of a polyatomic molecule. Answer: The number of atoms that constitute one molecule of an element is called its atomicity. Sulphur is a polyatomic molecule: S8 - 8 is the atomicity.

Question 38Question: What do the following stand for? (i) P4 and 4P (ii) O2 and 2O Answer:

P4 - 1 molecule of phosphorus 4P - 4 atoms of phosphorus O2 - 1 molecule of oxygen 2O - 2 atoms of oxygen

Question 39Question: How many gram atoms are present in 69 grams of sodium? Answer:

The number of gram atoms present in 69 grams of sodium is 3.

Question 40Question: The mass of a single atom of an element Z is 2.65x10-23g. What is its gram atomic mass? Answer: 1 atom of element Z has mass = 2.65 x 10-23g Hence, 6.023 x 1023 atoms of element Z have mass = 2.65 x 10-23 x 6.023 x 1023 = 15.69 g

Question 41Question: What is gram molecular mass? Answer: The amount of a substance whose mass in grams is numerically equal to its molecular mass is called gram molecular mass.

Question 42Question: What is the mass of 0.2 mole of lead nitrate? (N=14, O=16, Pb=207). Answer: Gram molecular weight of Pb(NO3)2 = 207 + (2 x 14) + 2(16 x 3) = 207 + 28 + 96 = 331

1 mole of Pb(NO3)2 is 331 g Therefore 0.2 mole of Pb(NO3)2 is 331 x 0.2 = 66.2 g

Question 43Question: Find the total percentage of oxygen in magnesium nitrate crystals i.e., Mg(NO3)2.6H2O (Atomic weight: H=1, N=14, O=16, Mg=24). Answer: Molecular weight of Mg(NO3)2.6H2O = 24 + 2(14 + 16 x 3) + 6(2 x 1 + 16) = 24 + 124 + 108 = 256 a.m.u Atomic mass of oxygen in Mg(NO3)2.6H2O is, = 2 (16 x 3) + 6 (16) = 96 + 96 = 192

Question 44Question: A compound has the following percentage composition H=2.04%, S=32.65%, O=65.31%. Relative molecular mass of the compound = 98. Calculate its molecular formula (H = 1, S = 32, O = 16). Answer: Empirical formula is H2SO4 Empirical formula mass = (2 x 1) + 32 + (16 x 4) = 98. Relative molecular mass = 98

Question 45Question: Calculate the amount of nitrogen supplied to soil by 1 quintal (100 kg) of ammonium nitrate (N=14, H=1, O=16). Answer: Molecular weight of NH4NO3 = 14 + (4 x 1) + 14 + (16 x 3) = 80 g

Molecular weight of N in the above formula = 14 x 2 = 28 80 units of NH4NO3 yield 28 units of Nitrogen.

Question 46Question: Identify diatomic molecules from the following: (i) HCl (ii) P4 (iii) He (iv) O3 (v) H2S (vi) CO Answer: HCl, CO are diatomic.