Ans: The laws of chemical combination include:-
I. Law of conservation of mass
II. Law of constant proportion
I. Law of Conservation of Mass:
It states that in a chemical reaction, mass can neither be created nor destroyed.
The mass of reactants is equall to mass of products.
CaCO3 _____> CaO + CO2
Cal. Carbonate cal oxide Carbon dioxide
100g 56 g 44g
II. Law of Constant proportion:
It states that in a chemical reaction, elements are always present in definite proportion by mass.
Example: Water (H2O), the ratio of mass of hydrogen to oxygen is 1:8 always.
In ammonia (NH3), The ratio of mass of nitrogen to hydrogen is 14:3 always.
Q. Write the postulates of daltons atomic theory?
Ans: Dalton proposed Atomic theory in 1808. The various postulates of Daltons atomic theory are:
a. All matter is made of tiny particles called atoms.
b. Atoms can neither be created nor destroyed.
c. Atoms of given element are identical in mass.
d. Atoms combine in the ratio of simple whole mumbers to form compounds.
e. The relative number of atoms are constant in given compound.
Q. Which postulate of daltons atomic theory explain law of conservation of mass?
Ans: Atoms can neither be created nor destroyed explains law of conservation of mass.
Q. Which postulate of daltons atomic theory explains Law of definite proportions?
Ans: Atoms combine in ratio of simple whole numbers to form compounds explains law of definite proportions.
Q. Define atom? How is the atom represented?
Ans: An atom is smallest part of element that takes part in chemical reactions.
Atoms of most elements are reactive and dont exist in free state.
Atoms are very small in size. They have nucleus containing protons and neutrons and shells where electrons are present.
The atoms are represented by one or two letters of its english or latin name.
The first letter is written as capital and second letter is small. The examples are:-
Hydrogen H Iron Fe
Carbon C Copper Cu
Oxygen O Silver Ag
Q. What is atomic mass of an element?
Ans: Atomic mass of the element is defined as relative mass of its atom with respect to carbon-12.
The actual mass of element is low, so we use relative mass with respect to C-12.
An atomic mass unit is defined one- twelfth of mass of carbon 12, which has been assigned atomic mass of 12 units.
Q. Define Molecule and Compound
Or
What are the molecules of element?
Ans:
Molecule: A molecule is electrically neutral group of two atoms bonded by covalent bond.
It is also defined as smallest particle which has properties of substance and exist in free state.
Compound: A compound is a chemical substance that contains two or more different atoms chemically combined togather.
Q. Define atomicity?
Ans: The number of atoms present in one molecule of element is called as atomicity.
The elements can be monoatomic , diatomic, triatomic etc.
The atomicity of metals and nobel gases is 1, therefore are monoatomic.
The atomicity of H2, N2, Cl2 is two, therefore are diatomic.
Q. What is an Ion? Write a note on Ionic compounds?
Ans: An Ion is a charged atom. The positive charged ion is called Cation, eg Na+. It is formed by loss of electrons.
The negative ion is callen anion, eg Cl-. It is formed by gain of electrons.
Ionic compounds: The compound formed by anions and cations is called ionic compound. They are held by ionic bond.
Eg, Na+ + Cl- ______> NaCl
Q. What is the molecular mass?
Ans: The molecular mass of the substance is its relative mass to mass of Carbon 12.
It indicates the number of times one molecule of substance is heavier than 1/12th of carbon 12.
Example: The molecular mass of H2 is 2U, The molecular mass of H2O is 18U.
Q. Define Mole?
Ans: The mole is link between mass of atom and number of atoms.
One Mole is defined as a number of
23
6.022 × 10 particles.
23
The number 6.022 × 10 is also called avogadros number.
One mole of substance contains same number of particles as in 12 grams of Carbon 12 element.
Textual Questions
Q. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
sodium carbonate + acetic acid → sodium acetate + carbon dioxide + water
Ans: According to the law of conservation of mass, the mass of products should be equal to mass of Products.
i.e. Mass of Reactants = Mass of Products
5.3 + 6 = 8.2 + 2.2 + 0.9
11.3 g = 11.3 g
LHS = RHS
This shows that the above observations are in agreement with the law of conservation of mass.
Q. Why is it not possible to see an atom with naked eyes?
Ans: The atoms do not exist independently and their size is very small, measured in nanometers i.e. 10^-9 m. Therefore it is not possible to see atoms with naked eyes.
Q. Hydrogen and oxygen combine in ratio of 1:8 by mass to form water. What mass of oxygen gas will react with 3 g of Hydrogen?
Ans: Ratio by which hydrogen and oxygen Mix= 1:8
i.e, 3 grams of hydrogen require= 8 × 3
= 24 g of O.
Q. Write the names of following compounds:
I. Al2(SO4)3 : Aluminium Sulphate
II. CaCl2 : Calcium Chloride
III. K2SO4 : Potassium Sulphate
IV. KNO3 : Potassium Nitrate
V. CaCO3 : Calcium Carbonate
Q. Write down the formulae of (i) sodium oxide (ii) aluminium chloride (iii) sodium suphide (iv) magnesium hydroxide
i) Sodium Oxide = Na2O
ii) Aluminium Chloride =AlCl3
iii) Sodium Sulphide = Na2S
iv) Magnesium Hydroxide = Mg(OH)2
Q. What is meant by the term chemical formula?
Ans: The chemical formula is the representation of chemical which contains the symbols and number of elements present in that chemical.
e.g. The chemical formula of Water is H2O. It contains chemical symbols of Hydrogen and Oxygen as well as the number of atoms of Hydrogen and oxygen present in it.
Q. How many atoms are present in a (i) H2S molecule and (ii) PO4 3– ion
Ans:
i) H2S: H2S means Hydrogen Sulphide and has 3 atoms. It contains 2 atoms of Hydrogen and 1 atom of Sulphur.
ii) PO4^ 3- : PO4^ 3- means Phosphate and has 5 atoms. It contains 4 atoms of Oxygen and 1 atom of Phosphorus.
Q. Calculate the molecular masses of H2 , O2 , Cl2 , CO2 , CH4 , C2H6 , C2H4 , NH3 , CH3OH.
Ans:
i) H2 => 1 x 2 = 2U.
ii) O2 => 16 x 2 = 32 U
iii) Cl2 => 35.5 x 2 = 71 U
iv) CO2 => 12 + (16 x2) =44 U
v) CH4 => 12 + (1 x 4) = 16 U
vi) C2H6 => (12 x 2) + (1 x 6) = 30 U
vii) C2H4 => (12 x 2) + (1 x 4) = 28 U
viii) NH3 => 14 + (1 x 3) = 17 U
ix) CH3OH => 12 + (1 x 3) + 16 + 1 = 32 U
Q. Calculate the formula unit masses of ZnO, Na2O, K2CO3 , (given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.)
Ans: Formula unit mass is the sum of atomic masses of all atoms in an ionic compound.
i) ZnO = 65 + 16 = 81 U
ii) Na2O = (23 x 2) + 16 = 62 U
iii) K2CO3 = (39 x 2) + 12 + (16 x 3) = 138 U
Q. Calculate molecular mass of CH4, NH3, CH3OH?
Ans:
Molecular Mass of Methane, CH4 =
Mass of carbon + 4 × Mass of Hydrogen
=> 12 + 4 × 1
=> 16 U
Molecular mass of Ammonia, NH3=
Mass of Nitrogen + 3 × Mass of Hydrogen
= 14 + 3 × 1
= 17 U
Molecular mass of Methanol, CH3OH=
Mass of Carbon + 3×Mass of Hydrogen + Mass of Oxygen + Mass of Hydrogen
= 12 + 3 × 1 + 16 + 1
= 32 U
Q. If one mole of carbon weights 12 g, What is the mass of 1 atom of carbon?
Ans:
1 mole of carbon = 12 g
23
1 mole = 6.022 × 10^ g
23
I.e 6.022 × 10 ^ = 12 g
Mass of 1 atom = 12
23
6.022 × 10
- 23
= 1.99 × 10 grams
Q. Which has more number of atoms, 100 g of sodium or 100 gram of Iron.
Ans: Atomic mass of sodium = 23 U
Molar mass of sodium = 23 g/mole
Given mass of sodium = 100 g
Therefore moles of Sodium= mass
Molar mass
= 100 = 4.34
23
Molar mass of Iron = 56 g/mole
Given mass of Iron = 100 g
Therefore moles of Iron= mass
Molar mass
= 56 = 1.78
100
Therefore 100 gm of Sodium has more atoms than 100 gm of iron.
Q. Calculate Molar mass of Ethylene, Phosphorus, Nitric acid.
Ans:
Ethylene = C2H2
Mass of 2 × carbon + 2 × Hydrogen
= 2 × 12 + 2 × 1
= 26 g/mol
Phosphorus = P4
Mass of 4 × Phosphorus
= 4 × 31
= 124 g/mol
Nitric Acid = HNO3
Mass of Hydrogen + Nitrogen + 3 × Oxygen
= 1 + 14 + 3 × 16
= 63 g/mol
Q. What is the mass of 4 moles of Aluminium atom?
Ans: Atomic Mass of Aluminium = 27 U
i.e , 1 mole of Aluminium = 27 grams
4 moles of Aluminium = 4 × 27 = 108 grams
Q. Convert 22 grams of CO2 into Moles.
Ans: 1 mole of CO2 = Mass of C + 2 × Mass of O
= 12 + 2 × 16 = 44 gram
Therefore, 22 grams of CO2 = 22/44 moles
= 0.5 moles
Q. What is the mass of 0.5 moles of Water?
Ans: Molar Mass of Water = 2 × Mass of H + Mass of O
= 2 × 1 + 16
= 18 grams
Therefore, Mass of 0.5 moles = 18 × 0.5
= 9 grams.
Q. 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.
Ans: Given,
Total Mass of Compound = 0.24g
Mass of Boron = 0.096 g
Mass of Oxygen = 0.144 g
We Know,
% composition = Mass of element x 100
Mass of Compound
Thus,
Mass of Boron = 0.096/0.24 x 100
Mass of Boron = 40%
Mass of Oxygen = Remaining 60%
Q. 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?
Ans: According to the law of Constant proportions, elements
are always present in definite proportion by mass.
In a carbon dioxide molecule, the atoms of carbon and oxygen
are present in the proportion of 3:8 by mass. This means 3 grams of Carbon will
react with 8 grams of Oxygen to form 11 grams of Carbon dioxide.
When 3.00 g of Carbon are burnt in 50.00 g of Oxygen, 3 g of
carbon will react with 8 g of Oxygen to form only 11.00 g of CO2.
42.00 grams of Oxygen will remain unused.
Q. What are polyatomic ions? Give examples.
Ans: Polyatomic ions are the Cations or Anions that contain
two or more than two atoms.
Examples: Ammonium Ion (NH4+), Sulphate ion (SO4^ 2-), Hydroxide Ion (OH-)
Q. Write the chemical formulae of the following. (a)
Magnesium chloride (b) Calcium oxide (c) Copper nitrate (d) Aluminium chloride
(e) Calcium carbonate.
(a) Magnesium chloride = MgCl2
(b) Calcium oxide =
CaO
(c) Copper nitrate = Cu(NO3)2
(d) Aluminium
chloride = AlCl3
(e) Calcium carbonate = CaCO3
Q. Give the names of the elements present in the following
compounds. (a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium
sulphate.
Ans:
(a) Quick lime = The formula of Quick lime is CaO. It contains
Calcium and Oxygen.
(b) Hydrogen bromide = The formula of Hydrogen bromide is
HBr. It contains Hydrogen and Bromine
(c) Baking powder = The
formula of Baking powder is NaHCO3. It contains Sodium, Hydrogen, Carbon and
Oxygen. Besides that it also contains Citric acid or tartaric acid.
(d) Potassium sulphate = The formula of Potassium sulphate
is K2SO4. It contains Potassium, Sulphur and Oxygen.
Q. What is the mass of 1 mole of Nitrogen atoms
Ans:
a. 1 mole of Nitrogen atoms:
Mass of 1 mole of Nitrogen atom = 1 x Mass of Nitrogen atom
= 1 x 14
= 14 g/mol
Q. What is the mass of 10 moles of Sodium sulphate
Ans:
10 moles of Sodium sulphate
Mass of 10 moles of Sodium sulphate = 10 x Molecular mass of
Sodium Sulphate (Na2SO4)
= 10 x [ (23 x 2) + 32 + (16 x 3)]
= 10 x [ 46 + 32 +42+ 48]
= 10 x 126
= 1260 g/mol
Q. Convert to moles,
a. 12 g of Oxygen atom
b. 20 g of Water
c. 22 g of Carbon dioxide
Ans:
a. 12 g of Oxygen atom:
Molecular Mass of Oxygen (O2) = 32 U
i.e. Mass of 1 mole of O2 gas = 32 g
Now,
Moles in 12 g of O2 gas = 12/32 = 0.37 Moles
b. 20 g of Water:
Molecular Mass of Water (H2O) = 18 U
i.e. Mass of 1 mole of H2O = 18 g
Now,
Moles in 20 g of H2O = 20/18 = 1.1 Moles
c. 22 g of Carbon dioxide:
Molecular Mass of Carbon Dioxide (CO2) = 44 U
i.e. Mass of 1 mole of CO2 = 44 g
Now,
Moles in 22 g of CO2 = 22/44 = 0.5 Moles
Q. What is the mass of 0.2 mole of oxygen atoms?
Ans:
We Know that,
Mass of 1 mole of Oxygen atom = 16 g
Thus, Mass of 0.2 moles of Oxygen atom = 0.2 x 16
= 3.2 g
Q. Calculate the number of molecules of Sulphur (S8) present in 16 g of solid Sulphur?
Ans: Given,
Mass of Sulphur = 16 g
Molecular Mass of Sulphur (S8) = 32 x 8 = 256 U
i.e. Mass of 1 mole of Sulphur = 256 g
i.e. Mass of 6.022 x 10^23 molecules of Sulphur = 256 g
Now, Molecules in 1 gram of Sulphur = 6.022 x 10^23 /256
Molecules in 16 g of S = 6.022 x 10^23
x 16
256
Molecules in 16 g of Sulphur = 3.76 x 10^22
Q. Calculate the number of aluminium ions present in 0.051 g of
aluminium oxide.
Ans: We Know that,
1 Mole of Aluminium oxide (Al2O3) = 2 x Mol mass of Aluminium + 3
x Mol mass of Oxygen
= (2 x 27) + (3 x 16)
Thus, Moles present in 0.051 g of Aluminium Oxide = 0.051/102 = 0.0005
Moles
Now,
1 Mole = 6.022 x 10^23 particles
Particles present in 0.0005 Moles = 0.0005 x 6.022 x 10^23
particles
i.e. 3.011 x 10^20 particles
Now, No. of Aluminium ions present in 1 molecule of Al2O3 = 2
Thus, No. of Aluminium ions present in 3.011 x 10^20 molecules = 2
x 3.011 x 10^20
= 6.022 x 10^20
