Download Atoms and Molecules

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Atoms and Molecules
During a chemical reaction, the sum of the masses of the reactants and products
remains unchanged. This is known as the Law of Conservation of Mass.
In a pure chemical compound, elements are always present in a definite proportion
by mass. This is known as the Law of Definite Proportions.
An atom is the smallest particle of the element that can exist independently and
retain all its chemical properties.
A molecule is the smallest particle of an element or a compound capable of
independent existence under ordinary conditions. It shows all the properties of the
substance.
A chemical formula of a compound shows its constituent elements and the number
of atoms of each combining element.
Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed
charge on them.
The chemical formula of a molecular compound is determined by the valency of each
element.
In ionic compounds, the charge on each ion is used to determine the chemical
formula of the compound.
Scientists use the relative atomic mass scale to compare the masses of different
atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass
of 12 and the relative masses of all other atoms are obtained by comparison with the
mass of a carbon-12 atom.
The Avogadro constant 6.022 × 1023 is defined as the number of atoms in exactly 12 g
of carbon-12.
The mole is the amount of substance that contains the same number of particles
(atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of
carbon-12.
Mass of 1 mole of a substance is called its molar mass.
Exercise:
1. What is Law of conservation of mass?
Answer: Law of conservation of mass states that mass can neither be created nor
destroyed in a chemical reaction, i.e. during a chemical reaction, the sum of the masses
of the reactants and products remains unchanged.
2. State the law of constant proportions. Explain with suitable example.
Answer: The law of constant proportions which is also known as the law of definite
proportions states that “In a chemical substance the elements are always present in
definite proportions by mass”.
For example: 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. Similarly in ammonia, nitrogen and hydrogen are
always present in the ratio 14:3 by mass, whatever the method or the source from
which it is obtained.
3. State Delton’s atomic theory.
Answer: According to Dalton’s atomic theory, all matter, whether an element, a
compound or a mixture is composed of small particles called atoms.
The postulates of this theory may be stated as follows:
(i) All matter is made 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.
4. 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 3g of hydrogen gas?
Answer: It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.
Then, the mass of oxygen gas required to react completely with 1g of hydrogen gas is 8g.
Therefore, the mass of oxygen gas required to react completely with 3g of hydrogen gas
is 8 × 3g = 24 g.
5. Which postulate of Dalton's atomic theory is the result of the law of conservation of
mass?
Answer: The postulate of Dalton's atomic theory which is a result of the law of
conservation of mass is
“Atoms are indivisible particles, which can neither be created nor destroyed in a
chemical reaction”.
6. Which postulate of Dalton's atomic theory can explain the law of definite
proportions?
Answer: The postulate of Dalton's atomic theory which can explain the law of definite
proportion is
“The relative number and kind of atoms in a given compound remains constant”.
7. 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, 0.9g 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
Answer: As stated, in a reaction, sodium carbonate reacts with ethanoic acid to
produce sodium ethanoate, carbondioxide, and water.i.e.
Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Now,
For Reactant:
Mass of sodium carbonate = 5.3g (Given)
Mass of ethanoic acid = 6g (Given)
For Product:
Mass of sodium ethanoate = 8.2g (Given)
Mass of carbon dioxide = 2.2 (Given)
Mass of water = 0.9g
(Given)
Now, total mass before the reaction = (5.3 + 6)g = 11. 3g
and total mass after the reaction = (8.2 + 2.2 + 0.9)g = 11.3g
Therefore, Total mass before the reaction = Total mass after the reaction
Hence, the given observations are in agreement with the law of conservation of
mass.
8. What is an atom?
Answer: An atom is the smallest particle of the element that can exist independently
and retain all its chemical properties.
9. Atomic radius is measured in ____________.(Fill in the blanks)
Answer: Nanometre. (1/10 9 m = 1 nm)
10. Write down the symbol for below mentioned atoms.
(i) Aluminium (ii) Copper (iii) Gold (iv) Silver (v) Lead (vi) Zinc (vii) Uranium (viii)
Cobalt.
Answer: (i) Aluminium - Al
(ii) Copper- Cu
(iii) Gold – Au
(iv)Silver – Ag
(v) Lead – Pb
(vi) Zinc—Zn
(vii) Uranium—U
(viii) Cobalt--- Co
11. What is atomic mass unit?
Answer: One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the
mass of one atom of carbon-12. It is written as 'u'.
12. What is relative atomic mass ?
Answer: 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.
13. Write down atomic masses of following elements:
Hydrogen, Carbon, Nitrogen, Oxygen, Sodium, Magnesium, Sulphur, Chlorine and
Calcium.
Answer:
Element
Hydrogen
Carbon
Nitrogen
Oxygen
Sodium
Atomic mass
1
12
14
16
23
Magnesium
Sulphur
Chlorine
Calcium
24
32
35.5
40
14. Why is it not possible to see an atom with naked eyes?
Answer: The size of an atom is so small that it is not possible to see it with naked eyes.
Also, atom of an element does not exist independently.
15. What is a Molecule?
Answer: A molecule is in general a group of two or more atoms that are chemically
bonded together, that is, tightly held together by attractive forces.
A molecule can be defined as the smallest particle of an element or a compound that is
capable of an independent existence and shows all the properties of that substance.
Atoms of the same element or of different elements can join together to form
molecules.
16. What is atomicity?
Answer: The number of atoms constituting a molecule is known as its atomicity.
For example, a molecule of oxygen consists of two atoms of oxygen and hence it is
known as a diatomic molecule, O2 that is its atomicity is 2.
17. Write the atomicity of the following molecules:
(i) H2SO4
(ii) CCl4
Answer: Atomicity is the number of atoms present in a molecule.
(i) H2SO4 = 7 atoms are present
(ii) CCl4 = 5 atoms are present.
18. What is an ion?
Answer: Compounds composed of metals and non metals contain charged species. The
charged species are known as ions.
19. What are anions and cations?
Answer: 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, a ‘cation’.
For example, take sodium chloride (NaCl). Its constituent particles are positively charged
sodium ions (Na+),cation and negatively charged chloride ions (Cl–),anion.
20. What is a chemical formula?
Answer: The chemical formula of a compound is a symbolic representation of its
composition.
21. What is valency ?
Answer: The combining power (or capacity) of an element is known as its valency.
22. What are polyatomic ions? Give examples.
Answer: A polyatomic ion is a group of atoms carrying a charge (positive or negative).
For example, ammonium ion ( 4 +), hydroxide ion (OH−), carbonate ion ( 3 2−),
sulphateion ( 4 2−).
23. Write down the formula for hydrogen chloride.
Answer: Formula of hydrogen chloride
Formula of the compound would be HCl.
24. Write down the formula of hydrogen sulphide.
Answer: Formula of hydrogen sulphide
25. Write down the formula of ammonium sulphate.
Answer: Formula of ammonium sulphate:
26. Write down the formula of sodium carbonate.
Answer: Formula of sodium carbonate.
27. Write down the formulae of (i) sodium oxide (ii) aluminium chloride (iii) sodium
suphide(iv) magnesium hydroxide.
Answer: (i) Sodium oxide → Na2O
(ii) Aluminium chloride → AlCl3 (since valency of Aluminium is +3)
(iii) Sodium suphide → Na2S
(since valency of sulphide is -2)
(iv) Magnesium hydroxide → Mg(OH)2 (since valency of Magnesium is +2)
28. Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3.
Answer:
(i) Al2(SO4)3→ Aluminium sulphate
(ii) CaCl2→ Calcium chloride
(iii) K2SO4→ Potassium sulphate
(iv) CaCO3→ Calcium carbonate
29. How many atoms are present in a (i) H2S molecule and (ii) PO43– ion?
Answer: (i) In an H2S molecule, three atoms are present; two of hydrogen and one of
sulphur.
(ii) In a PO43- ion, five atoms are present; one of phosphorus and four of oxygen.
30. What is Molecular mass?
Answer: The molecular mass of a substance is the sum of the atomic masses of all the
atoms in a molecule of the substance. It is therefore the relative mass of a molecule
expressed in atomic mass units (u).
31. (a) Calculate the relative molecular mass of water (H2O).
(b) Calculate the molecular mass of HNO3.
Answer: (a) Atomic mass of hydrogen = 1u, oxygen = 16 u
So the molecular mass of water, which contains two atoms of hydrogen and one
atom of oxygen is = 2 × 1+ 1×16 = 18 u.
(b) The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N+ 3 ×
the atomic mass of O = 1 + 14 + 48 = 63 u.
32. What is formula unit mass? Explain with an example.
Answer: The formula unit mass of a substance is a sum of the atomic masses of all atoms
in a formula unit of a compound. Formula unit mass is calculated in the same manner as
we calculate the molecular mass. The only difference is that we use the word formula
unit for those substances whose constituent particles are ions. For example, sodium
chloride as discussed above, has a formula unit NaCl. Its formula unit mass can be
calculated as–
1 × 23 + 1 × 35.5 = 58.5 u
33. Calculate the formula unit mass of CaCl2.
Answer: Atomic mass of Ca + (2 × atomic mass of Cl) = 40 + 2 × 35.5 = 40 + 71 = 111 u.
34. Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Answer: Molecular mass of H2 = 2 × Atomic mass of H
= 2 × 1 = 2u
Molecular mass of O2 = 2 × Atomic mass of O
= 2 × 16 = 32u
Molecular mass of Cl2 = 2 × Atomic mass of Cl
= 2 × 35.5 = 71 u
Molecular mass of CO2 = Atomic mass of C + 2 × Atomic mass of O
= 12 + 2 × 16
= 44 u
Molecular mass of CH4 = Atomic mass of C + 4 × Atomic mass of H
= 12 + 4 × 1
= 16 u
Molecular mass of C2H6 = 2 × Atomic mass of C + 6 × Atomic mass of H
= 2 × 12 + 6 × 1 = 30u
Molecular mass of C2H4 = 2 × Atomic mass of C + 4 × Atomic mass of H
= 2 × 12 + 4 × 1 = 28u
Molecular mass of NH3 = Atomic mass of N + 3 × Atomic mass of H
= 14 + 3 × 1 =17 u
Molecular mass of CH3OH Atomic mass of C+4 ×Atomic mass of H+ Atomic
mass of O
= 12 + 4 × 1 + 16 = 32 u
35. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given masses of Zn = 65u,
Na = 23u, K = 39u, C = 12u, and O = 16u.
Answer: Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 + 16 = 81 u
Formula unit mass of Na2O = 2 × Atomic mass of Na + Atomic mass of O
= 2 × 23 + 16 = 62u
Formula unit mass of K2CO3 = 2 × Atomic mass of K + Atomic mass of C + 3 ×
Atomic mass of O
36. What is mole?
= 2 × 39 + 12 + 3 × 16 = 138u
Answer: One mole of any species (atoms, molecules, ions or particles) is that quantity in
number having a mass equal to its atomic or molecular mass in grams.
37. What is Avogadro Number?
Answer: The number of particles (atoms, molecules or ions) present in 1 mole of any
substance is fixed, with a value of 6.022 × 1023. This is an experimentally obtained value.
This number is called the Avogadro Constant or Avogadro Number (represented by N 0),
named in honour of the Italian scientist, Amedeo Avogadro.
38. Calculate the number of moles for the following:
(i) 52 g of He (finding mole from mass)
(ii) 12.044 × 1023 number of He atoms (finding mole from number of particles).
Answer: No. of moles = n
Given mass = m
Molar mass = M
Given number of particles = N
Avogadro number of particles = N0
39. Calculate the mass of the following:
(i) 0.5 mole of N2 gas (mass from mole of molecule)
(ii) 0.5 mole of N atoms (mass from mole of atom)
(iii) 3.011 × 1023 number of N atoms (mass from number)
(iv) 6.022 × 1023 number of N2 molecules (mass from number)
Answer:
40. Calculate the number of particles in each of the following:
(i) 46 g of Na atoms (number from mass)
(ii) 8 g O2 molecules (number of molecules from mass)
(iii) 0.1 mole of carbon atoms (number from given moles)
Answer:
41. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom
of carbon?
Answer: One mole of carbon atoms weighs 12g
(Given)
i.e., mass of 1 mole of carbon atoms = 12g
Then, mass of 6.022× 1023 number of carbon atoms = 12g
Therefore, mass of 1 atom of carbon =
.
g
= 1.9926 × 10– 23 g
42. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given,
atomic mass of Na = 23u, Fe =56 u)?
Answer: Atomic mass of Na = 23u
(Given)
Then, gram atomic mass of Na = 23g
Now, 23g of Na contains = 6.022×1023 number of atoms
Thus, 100g of Na contains =
.
number of atoms
= 2.6182 × 1024 number of atoms.
Again, atomic mass of Fe = 56u (Given)
Then, gram atomic mass of Fe = 56g
Now, 56 g of Fe contains = 6.022×1023 number of atoms
Thus, 100 g of Fe =
.
number of atoms
= 1.0753 × 1024 number of atoms
Therefore, 100 grams of sodium contain more number of atoms than 100 grams of iron.
Multiple Choice Questions:
1. Which of the following correctly represents 360 g of water?
(i) 2 moles of H20
(ii) 20 moles of water
(iii) 6.022 × 1023 molecules of water
(iv) 1.2044×1025 molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
2. Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently
(b) Atoms are the basic units from which molecules and ions are formed
(c) Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or
touch
3. The chemical symbol for nitrogen gas is
(a) Ni
(b) N2
(c) N+
(d) N
4. The chemical symbol for sodium is
(a) So
(b) Sd
(c) NA
(d) Na
5. Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C12 H22 O11)
(b) 2 moles of CO2
(c) 2 moles of CaCO3
(d) 10 moles of H2
6. Which of the following has maximum number of atoms?
(a) 18g of H2O
(b) 18g of O2
(c) 18g of CO2
(d) 18g of CH4
7. Which of the following contains maximum number of molecules?
(a) 1g CO2
(b) 1g N2
(c) 1g H2
(d) 1g CH4
8. Mass of one atom of oxygen is
9. 3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of oxygen
atoms in the solution are
(a) 6.68 × 1023
(b) 6.09 × 1022
(c) 6.022 × 1023
(d) 6.022 × 1021
10. A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
11. Which of the following represents a correct chemical formula?
(a) CaCl
(b) BiPO4
(c) NaSO4
(d) NaS
Answer:
11. (b) BiPO4 — Both ions are trivalent. Bismuth phosphate