Goa Board Class 9 Solutions for Chemistry – Atoms and Molecules (English Medium)
Page No. 32:
Question 1:
In 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?
Sodium carbonate + ethanoic acid –> sodium ethanoate + carbon dioxide + water
Solution:
The reactants in this reaction are sodium carbonate and ethanoic acid and the products are carbon dioxide, water and sodium ethanoate.
The total mass of the reactants = 5.3 + 6 = 11.3 g
The total mass of the products = 2.2 + 0.9 + 8.2 = 11.3 g
Thus, the total mass of the reactants is same as that of the products. Thus, the mass was neither created nor destroyed during the chemical reaction. Therefore, the mass was conserved during the reaction of sodium carbonate and ethanoic acid.
Concept insight: Firstly, recall the law of conservation of mass. And then check whether it is being followed by the data which is given or not.
Page No. 33:
Question 2:
Hydrogen 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?
Solution:
The law of constant proportions states that in a chemical substance, the elements are always present in definite proportions by mass. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water.
1g of hydrogen reacts with oxygen = 8g
Thus, in order to form water, 3 x 8 = 24 g of oxygen will react completely with 3 g of hydrogen gas.
Concept insight: Recall the law of constant proportions and then apply that in a chemical substance, the elements are always present in definite proportions by mass.
Question 3:
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Solution:
The second postulate of Dalton’s atomic theory which states that “Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction” is the result of the law of conservation of mass.
Concept insight: For answering this question you should know both the law of conservation of mass and the postulates of Dalton’s atomic theory.
Question 4:
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Solution:
The postulate of Dalton’s atomic theory which states that “The relative number and kinds of atoms are constant in a given compound” can explain the law of definite proportions.
Concept insight: For answering this question, you should know both law of definite proportions and the postulates of Dalton’s atomic theory.
Page No. 35:
Question 1:
Define the atomic mass unit.
Solution:
Atomic mass unit is a mass unit equal to exactly one-twelfth the mass of a carbon-12 atom. It is used to measure the masses of subatomic particles.
Concept insight: In this answer, write the proper definition of atomic mass unit.
Question 2:
Why is it not possible to see an atom with naked eyes?
Solution:
The dimension of atoms is very, very small and is of the order of 1 nm. Thus, we cannot see an atom with naked eyes.
Concept insight: For answering this question, you should recall the size of an atom.
Page No. 39:
Question 1:
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Solution:
Concept insight: For answering this question, write the cation and anion with the proper charges, cross multiply and write the formula.
Question 2:
Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3
Solution:
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate
Concept insight: For writing the name of the compound, write the name of the cation first and then the anion.
Question 3:
What is meant by the term chemical formula?
Solution:
The chemical formula of a compound is a symbolic representation of its composition.
Concept insight: For this question, write the proper definition of chemical formula.
Question 4:
How many atoms are present in a
(i) H2S molecule and
(ii) PO43- ion?
Solution:
(i) Three (Two hydrogen atoms and one sulphur atom)
(ii) Five (One phosphorus and four oxygen)
Concept insight: Count the total number of atoms of each type and add them.
Page No. 40:
Question 1:
Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Solution:
Molecular mass of H2 = Mass of 2H atoms = 1u + 1u = 2u
Molecular mass of O2 = Mass of 2O atoms = 16u + 16u = 32u
Molecular mass of Cl2 = Mass of 2Cl atoms = 35.5u + 35.5u = 71u
Molecular mass of CO2 = Mass of C atom + 2 x Mass of O atom = 12u + (2 � 16u) = 44u
Molecular mass of CH4 = Mass of C atom + 4 x Mass of H atom = 12u + (4 � 1 u) = 16u
Molecular mass of NH3 = Mass of N atom + 3 x Mass of H atom = 14u + (3 � 1 u) = 17u
Molecular mass of CH3OH = Mass of C atom + 3 x Mass of H atom + Mass of O atom + Mass of H atom = 12u + (3 � 1 u) + 16 u + 1 u = 32u
Concept insight: For calculating the molecular mass, add the atomic masses of all the atoms present in the formula.
Question 2:
Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65u, Na = 23u, K = 39u, C = 12u, and O = 16u.
Solution:
Formula unit mass of ZnO = 65 u + 16 u = 81 u
Formula unit mass of Na2O = (2 x 23 u) + 16 u = 62 u
Formula unit mass of K2CO3 = (2 x 39 u) + 12 u + (3 x 16 u) = 138 u
Concept insight: For calculating the formula unit mass, add the atomic masses of all the atoms present in the formula.
Page No. 42:
Question 1:
If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Solution:
1 mole = 6.022 x 1023 atoms
One mole of carbon atoms weighs 12 gram.
i.e. 6.022 x 1023 atoms of carbon weigh 12 gram.
Therefore,
1 atom of carbon weighs 12 / (6.022 x 1023) = 1.993 x 10-23 gram.
Concept insight: For answering questions related to mole concept apply the relation
Question 2:
Which has more number of atoms, 100 grams of sodium or 100 grams of iron (Given, atomic mass of Na = 23 u, Fe = 56 u)?
Solution:
Page No. 43:
Question 1:
A 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.
Solution:
Question 2:
When 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 combination will govern your answer?
Solution:
3.0 g of carbon combines with 8.0 g of oxygen to give 11.0 of carbon dioxide.
Thus, by the law of definite proportions, when 3.00 g of carbon is burnt in 50.00 g of oxygen, only 8.00 g of oxygen will be used to produce 11.00 gram of carbon dioxide. The remaining 42.00 g of oxygen will remain unreacted. Law of constant proportion is governed.
Concept insight: Relate the given data in the question to the laws and then identify which law is governing the answer.
Page No. 44:
Question 3:
What are polyatomic ions? Give examples.
Solution:
Polyatomic ions are those ions that are formed from groups of joined atoms. These atoms can be of the same type or of different type. Some examples of polyatomic ions are NO3–, NH4+, OH–, SO42-, and SO32-.
Concept insight: Recall the definition of polyatomic ions and give examples of it. It is an important question from exam point of view.
Question 4:
Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Solution:
(a) MgCl2
(b) CaO
(c) Cu(NO3)2
(d) AlCl3
(e) CaCO3
Concept insight: For answering this question, write the cation and anion with the proper charges, cross multiply and write the formula.
Question 5:
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Solution:
(a) Calcium and oxygen
(b) Hydrogen and bromine
(c) Sodium, hydrogen, carbon, and oxygen
(d) Potassium, sulphur, and oxygen
Concept insight: For answering this question, write the formula of the compound first, see which all atoms are present in it and then write their names.
Question 6:
Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Solution:
(a) Molecular mass of ethyne, C2H2 = 2 x Mass of C + 2 x Mass of H = 2 x 12u + 2 x u = 26u
(b) Molecular mass of sulphur molecule, S8 = 8 x Mass of S = 8 x 32u = 256u
(c) Molecular mass of phosphorus molecule, P4 = 4 x Mass of P = 4 x 31u = 124u
(d) Molecular mass of HCl = Mass of H + Mass of Cl = 1 u + 35.5u = 36.5u
(e) Molecular mass of HNO3 = Mass of H + Mass of N + 3 x Mass of O = 1 u + 14u + 3 x 16u = 63u
Concept insight: For calculating the molecular mass, add the atomic masses of all the atoms present in the formula.
Question 7:
What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?
Solution:
(a) Mass of one mole of nitrogen atoms = molecular mass of nitrogen atoms in grams = 14 g
(b) Mass of 4 moles of aluminium atoms = 4 x Mass of 1 mole of Al atoms = 4 x molecular mass of aluminum atoms in grams = 4 x 27 = 108 g
(c) Mass of 10 moles of sodium sulphite = 10 x Mass of 1 mole of Na2SO3 = 10 x molecular mass of Na2SO3 in grams
Molecular mass of sodium sulphite, Na2SO3 = 2 x 23 + 32 + 3 x 16 = 126 g
Thus, the mass of 10 moles of sodium sulphite = 10 x 126 = 1260 g
Concept insight: For answering questions related to mole concept, apply the relation
For an atom ,
1 mole = Gram atomic mass = 6.022 x 1023 atoms
For a molecule,
1 mole = Gram atomic mass = 6.022 x 1023 molecules
Question 8:
Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide.
Solution:
Question 9:
What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Solution:
(a) The mass of 1 mole of oxygen atoms = 16 g
Thus, the mass of 0.2 mole of oxygen atoms = 16 x 0.2 = 3.2 g
(b) The mass of 1 mole of water molecules = 18 g
Thus, the mass of 0.5 mole of water molecules = 18 x 0.5 = 9.0 g
Concept insight: For answering questions related to mole concept, apply the relation:
For an atom,
1 mole = Gram atomic mass = 6.022 x 1023
For a molecule,
1 mole = Gram molecule mass = 6.022 x 1023
Question 10:
Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur
Solution:
Concept insight: For answering questions related to mole concept, apply the relation:
For an atom,
1 mole = Gram atomic mass = 6.022 x 1023
For a molecule,
1 mole = Gram molecule mass = 6.022 x 1023
Question 11:
Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Solution:
1 mole = Gram atomic mass = 6.022 x 1023
For a molecule,
1 mole = Gram molecule mass = 6.022 x 1023