"AKC SCIENCE CLASSES"
CLASS 09 TH (CBSE AND MP BOARD)
CHAPTER 03
ATOMS AND MOLECULES
INTEXT QUESTIONS AND ANSWERS
NCERT INTEXT QUESTIONS (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
Ans:-
Law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. It means the mass remains the same. So, we add the mass of the reactants on LHS and add the mass of all products on RHS
LHS = 5.3 g + 6 g = 11.3 g
RHS = 8.2g + 2.2g + 0.9g = 11.3g
LHS = RHS
So, the observations are in agreement with the law of conservation of mass.
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?
Ans :- 'Law of constant proportions' states that composition of a compound is always fixed. Applying this
∵ 1 g of hydrogen gas combines with oxygen = 8 g
∴ 3 g of hydrogen gas will combine with oxygen = 8 x 3 = 24 g
Question 3:- Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
Ans:- Following postulate of Dalton's atomic theory is the result of the law of conservation of mass. 'Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.'
Question 4:- Which postulate of Dalton's atomic theory can explain the law of definite proportions?
Ans:- Following postulate of Dalton's atomic theory can explain the
'Law of definite proportions'.
'The relative number and kinds of atoms are constant in a given compound.'
NCERT INTEXT QUESTIONS (PAGE NO. 35)
Question 1:- Define the atomic mass unit.
Ans:- One atomic mass unit (amu) is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.' The relative atomic masses of all the elements have been found with respect to an atom of carbon-12.
Question 2:- Why is it not possible to see an atom with naked eyes?
Ans:- As an atom is extremely small in size, it is not possible to see it with naked eyes. Generally radius of an atom is of the order of nanometres. For example, atomic radius of hydrogen atom is 10-10m (or 10-1nm).
NCERT INTEXT QUESTIONS (PAGE NO. 39)
Question 1:- Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium sulphide
(iv) magnesium hydroxide
Ans:-
(i)Na2O
(ii)NH4Cl
(iii)Na2SO4
(iv)Mg(OH)2
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
Ans:-
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate.
Question 3:- What is meant by the term chemical formula?
Ans:- Chemical formula of a compound (or element) is the symbolic representation of its composition. It represents
(i) The number and kind of atoms present per molecule of the compound,
(ii) One mole of the compound,
(iii) Molar mass of the compound.
Question 4:- How many atoms are present in a
(i) H2S molecule and (ii) PO4³⁻ ion?
Ans:-
(i) 2 atom of hydrogen + 1 atom of sulphur
= three (3) atoms (in a H2S molecule).
(ii) 1 atom of phosphorus + 4 atoms of oxygen
= five (5) atoms (in a PO4³⁻ ion).
NCERT INTEXT QUESTIONS (PAGE NO. 40)
Question 1:- Calculate the molecular masses of
H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Ans:-
(i) Molecular mass of H2 (hydrogen)
= Atomic mass of hydrogen × 2
= 1 × 2 = 2 u
(ii) Molecular mass of O2 (oxygen)
= Atomic mass of oxygen × 2
= 16 × 2 = 32 u
(iii) Molecular mass of CI2 (chlorine)
= Atomic mass of chlorine × 2
= 35.5 × 2 = 71 u
(iv) Molecular mass of CO2 (carbon dioxide)
= (Atomic mass of carbon × 1)+ (Atomic mass of oxygen × 2)
= 12 + (16 × 2) = 12 + 32 = 44 u
(v) Molecular mass of CH4 (methane)
= (Atomic mass of carbon × 1) + (Atomic mass of hydrogen × 4)
= 12 + (1 × 4) = 12 + 4 = 16 u
(vi) Molecular mass of C2H6 (ethane)
= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 6)
= (12 × 2) + (1 × 6) = 24 + 6 = 30 u
(vii) Molecular mass of C2H4 (ethene)
= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 4)
= (12 × 2) + (1 × 4) = 24 + 4 = 28 u
(viii) Molecular mass of NH3 (ammonia)
= (Atomic mass of nitrogen × 1) + (Atomic mass of hydrogen × 3)
= (14 × 1) + (1 × 3) = 14 + 3 = 17 u
(ix) Molecular mass of CH3OH (methanol or methyl alcohol)
= (Atomic mass of carbon × 1) + (Atomic mass of hydrogen × 3) + (Atomic
mass of oxygen × 1) + (Atomic mass of hydrogen × 1)
= 12 + 3 + 16 + 1 = 32 u
Question 2:- Calculate the formula unit masses of Zn0, Na20, K2C03. Given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u and O = 16 u.
Ans:-
(i) Formula unit mass of ZnO (zinc oxide) =
65 + 16 = 81 u
(ii) Formula unit mass of Na2O (sodium oxide) =
(23 × 2) + (16 × 1) = 46 + 16 = 62 u
(iii) Formula unit mass of K2CO3(potassium carbonate) =
(39 × 2) + (12 × 1) + (16 × 3) = 78 + 12 + 48 = 138 u
NCERT INTEXT QUESTIONS (PAGE NO. 42)
Question 1:- If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?
Ans:-
1 mole carbon atom = 6.022 x 10⁻²³ atoms
Molar atomic mass = 12 g
6.022 x 1023 carbon atoms weigh = 12 g
12
1 carbon atom weighs ------------ = 1.99 x 10⁻²³g
6.022 x 10²³
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)?
Ans:-
1). Number of moles of Na
Mass of Na in grams 100
= ------------------------- = ------ = 4.3
Gram atomic mass of Na 23
2). Number of moles of Fe
Mass of Fe in grams 100
= ----------------------- = ----- = 1.8
Gram atomic mass of Fe 56
We find that 100g of Na contains more moles of atoms.
