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Atom is the smallest particle of matter that can take part in a chemical reaction.
Atom is not capable of independent existence.
Two or more atoms combine together to form molecules.
By observing the laws of chemical combination discussed above, John Dalton (1808) proposed atomic theory of matter. The main points of Dalton’s atomic theory are as follows:
The main failures of Dalton’s atomic theory are:
Molecule can be defined as the smallest particle of matter which is capable of independent existence. A molecule is assembly of two or more atoms bonded together tightly.
Molecules can be classified into two groups depending on the type the elemental atoms.
Homoatomic Molecules:These are the molecules of elements and are made up of one type of atoms only Examples : H2, O2, N2, S8
Heteroatomic Molecules: These are the molecules of compounds and are made up of more than one type of elements. Examples : NH3, H2O, CH4
As atoms are very tiny particles, their absolute masses are difficult to measure. However it is possible to determine the relative masses of different atoms if small unit of mass is taken as standard (previously, this standard was mass of one atom of hydrogen and taken as unity. Later on it was part of oxygen atom and now it is part of C-12 atom).
The atomic mass of an element can be defined as the number which indicates how many times the mass of one atom of the element is heavier in comparison to the mass of one atom of hydrogen.
Atomic mass of an element
Atomic mass is not absolute but relative to the weight of the standard reference element C-12.
Atomic Mass Unit: The quantity mass of an atom of carbon-12 is known as the atomic mass unit and is abbreviated as amu. The actual mass of one atom of carbon-12 is or
Thus 1 amu
Actual mass of an element
Molecular weight is the number of times a molecule of any compound is heavier than 1/2th of an atoms of C-12.
Molecular weight of an element =
Molecular weight is calculated by adding the atomic weights of all the atoms present in a molecule.
Example, Molecular weight of water (H2O) = 2 x Atomic Weight of H + Atomic Weight of Oxygen = 2 x 1 + 16 = 18.
Atomic Weight is determined by Dulong and Petit method.
It states that “The product of atomic weights and specific heat of an element is 6.4 approximately.”
Atomic Weight x Specific Heat = 6.4
Step 1: Atomic weight (app.) = 6.4/Specific heat
Step 2: Valency = Atomic weight / Equivalent weight
Step 3: Correct atomic weight = Equivalent weight x Valency
Elements are found in different isotopic forms (atoms of same elements having different atomic mass), so, one can say that atoms of same elements which have same atomic numbers but different mass numbers are called isotopes.
In case of isotopes, atomic mass of the element is average of relative masses of different isotopes of the element.
Average atomic mass
There are two isotopes of chlorine Cl35 and Cl37.
Isotopes
Relative Masses
Relative Abundance (RA)
Cl35
35
3
Cl37
37
1
Atomic Mass of Chlorine = (Atomic mass of Cl35 x3) +(Atomic mass of Cl37 x1)/ (3+1) = 35.3
When atomic mass of any element is reported in grams, it is called Gram Atomic Mass( GAM)
1 GAM of Na = 23 gm of Na
1 GAM of Ca = 40 gm of Na
Similarly Carbon has two isotopes C-12 and C-13.
Refer to the following video for average atomic mass
The product of specific heat of pure element and atomic mass of the element is equal to 6.4.
i.e. Atomic mass ´ specific heat = 6.4 (approx)
But this law is not applicable to lighter element like boron, carbon, silicon. To obtain correct atomic mass of element first of all equivalent mass of the element is known by any other method and their atomic mass = eq. weight ´ valency
In which valency has whole number value which can be deduced by dividing approximate by equivalent mass.
Dulongs and Petit’s Law:
Solved Example
Question: 1
The specific heat of a metal of atomic mass 32 is likely to be:
a. 0.25
b. 0.24
c. 0.20
d. 0.15
Solution:
Specific heat =
Hence (C) is correct.
_______________________________________________
Question: 2
On dissolving 6 gm of metal in sulphuric acid, 13.53g of the metal sulphate was formed. The specific heat of metal is. What is equivalent mass of metal, valency and exact atomic mass?
Equivalent mass of sulphate
Eq. weight of metal
Approximate atomic mass
Valency
Exact atomic mass
If an element has several compound with other same or different elements of known atomic mass then the compound that has minimum presence of former element indicate the atomic mass of former element.
Procedure
Question
Estimate the atomic mass of nitrogen given that vapour density of NH3 = 8.5, Nitrous oxide = 22, Nitric oxide = 15, Nitrogen peroxide = 23, Nitrogen
trioxide = 38.
Compound
V.D.
Molecular weight of compound
Nitrogen mass in compound
Least presence of nitrogen
Ammonia
8.5
17
14
So atomic mass of nitrogen = 14
Nitrous oxide
22
44
28
Nitric oxide
15
30
Nitrogen peroxide
23
46
Nitrogen trioxide
38
76
When two or more compound forms similar type of crystals or able to form mixed crystals, they are known as isomorphs. For examples: MgSO4.7H2O, ZnSO4.7H2O and FeSO4.7H2O are isomorphs of each other as their crystals posses same shape.
According to Mitscherlich [year 1819].
The valency of elements that are similarly placed to that of other elements in their isomorphs are always same.
In the above example Fe, Zn and Mg have same valency [2] and equal ratio of water molecule in each isomorphs.
If equivalent mass of one element is known then atomic mass can be calculated by knowing the valency of other isomorphs key element.
Which pair of the following substances is said to be isomorphous?
a. White vitriol and blue vitriol
b. Epsom salt and Glauber’s salt
c. Blue vitriol and Glauber’s salt
d. White vitriol and Epsom salt
Solution
Epsom salt (MgSO4.7H2O) and White vitriol (ZnSO4.7H2O) contains divalent cation Mg2+ and Zn2+ and same number of water molecules as water of crystallization which hold criteria for isomorphism.
Hence (D) is correct.
The following steps are involved in this method
Let the valency of the element be x. The formula of its chloride will be
Molecular weight of chloride = Atomic mass of M + 35.5 x
So
Vapour Density = (Weight of certain volume of gas or vapour under certain temperature and pressure)/( Weight of the same volume of H2 under same temperature and pressure)
Molecular Weight = 2 x Vapour Density
r ∝ P at constant temperature
r ∝ 1 / √d at constant temperature
When molecular mass of a compound is expressed in grams, it is called Gram Molecular mass (GMM) or Gram Molecular weight (GMW).
Reactants, often yield quantities of products that are less than those calculated from the balanced chemical equation.
Reason behind such discrepancy may be:
So percentage yield is the ratio of actual yield (recovered) to theoretical yield multiplied by 100.
Question 1: Atomic mass is the number of times an atom of any element is heavier than 1/2th of an atoms of
a. C-12.
b. C-13
c. Cl -35
d. Cl -37
Question 2: What would be the molecular mass of a compound whose vapour density is 5 ?
a. 5
b. 10
c. 15
d. 20
Question 3: What is the molecular weight of HNO3?
( Given: Atomic weight of N, H and O are 14, 2 and 16 respectively)
a. 32
b. 64
c. 46
d. 23
Question 4: Isotopes of any element have
a. same atomic weight but different atomic number.
b. same atomic weight and atomic number.
c. different atomic weight but same atomic numbers.
d. different atomic weight and atomic numbers.
Q.1
Q.2
Q.3
Q.4
a
b
c
You can also refer to syllabus of chemistry for IIT JEE
Look here for the past year papers of IIT JEE
Click here to refer states of matter
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