Molar Mass

Two very important entities of elements that recognise its characteristics are its atomic number and atomic mass. Similarly, mol mass of a compound is very important and is used in almost all stoichiometric calculations of a compound.

Molar Mass Definition

Molecular mass of a compound is the sum of atomic masses of all elements present in it.

Molecular mass is a physical property of a compound. It is denoted by M.

Molar Mass-

Molecular mass can also be calculated using the mass of a substance and the amount of substance present in moles.

Molecular mass = Grams of a substance / Moles

Mol mass is expressed in grams per mole.

Mol mass is used to find the moles of a substance, when its mass is given.

How to Calculate Molar Mass –

Molar mass of a substance can be found from the atomic masses of the elements present in it. Atomic masses of the individual elements can be obtained from the periodic table.

To calculate mol mass, we need to follow the following steps:

1. Write the Formula for Molar Mass of the compound whose mol mass is to be calculated.

Example – Magnesium chloride

MgCl₂

2. Find the subscripts/number of each element present in the compound.

1 x Mg + 2 x Cl

3. Multiply the number of each element present with the atomic mass of that element.

1 x 24.305 (Mg) 2 x 35.453 (Cl)

= 24.305 = 70.906

4. Finally, put all the atomic masses and their multiplied values together and sum it up.

Mg + 2 x Cl

= 24.305 + 70.906

= 95.211 grams/mole

Molar Mass of Water –

Formula of water is H₂O.

To calculate the mol mass of water, we need to have the atomic masses of hydrogen and oxygen.

Atomic mass of hydrogen = 1.008 grams/mole

Atomic mass of Oxygen = 15.994 grams /mole

There are two moles of Hydrogen. Thus, 2 x 1.008 = 2.016

Mol mass of water = 2.016 + 15.994 = 18.01 grams/mole.

Example of molar mass calculations:

To calculate the mol mass of Sodium hydroxide:

Formula of sodium hydroxide is NaOH

There are 1 mole of sodium, Na, 1 mole of hydrogen, and 1 mole of Oxygen.

To find the mol mass, we can add the atomic masses of all the elements.

Atomic mass of Sodium = 22.989 grams/mole

Atomic mass of oxygen = 15.994 grams/mole

Atomic mass of Hydrogen = 1.008 grams/mole

Mol mass of Sodium hydroxide = Na + O + H

= 22.989 + 15.994 + 1.008 = 39.991 grams/mole.

Mol mass of sodium hydroxide is taken approximately for calculations as 40 grams/mole.

Metals Nonmetals and Metalloids

Periodic table consists of an array of elements, with different metallic properties. Some are completely metallic, making them soft or hard metals. Other major type includes the non-metals, which have completely different physical and chemical properties from metals and are easily distinguishable.

Some elements have properties in-between that of a metal and a non-metal. These types of elements are termed as ‘metalloids’.

Characteristics of Metals Nonmetals Metalloids-

Common characteristics through which a metal, non-metal and a metalloid can be differentiated are:

Metals:

An element is called as metal, when, in the process of forming an ionic bond, it donates electrons, to form a positive ion. Thus, the main characteristics of a metal is that, it should have very less first ionization energy, or energy due to removal of outermost electron.

Some common properties of metal are:

1. Metals are mostly solids, hard or soft. Metals of first two groups of the periodic table are soft solids, while transition metals are hard. Mercury is the only metal, which is a liquid.
2. They are malleable and ductile.
3. They transfer heat and electricity due to the presence of ions in their structure. There is a special type of bond called as metallic bond, which gives metals all these distinctive properties.

Non-metals:

An element is said to be a non-metal, when it shows electronegative property than electropositive property. They accept electrons to form an ionic bond. Their first ionization energy is very high.

1. Non-metals are mostly liquids, gases or in some case, amorphous solids.
2. If they are solids, they are brittle solids, and are not malleable and ductile.

Metalloids:

These are elements which have properties in-between that of metals and non-metals. Metalloids are called as semi-metals. They have lustre like metals, but do not conduct electricity.

Metalloids find use as semiconductors. Metalloids are placed with the non-metals in 14^th, 15^th and 16^th Group of the periodic table.

List of Metals Nonmetals and Metalloids

Some of the metals, metalloids and non-metals are listed below:

Metals	Metalloids	Non-metals
Copper- Cu	Silicon -Si	Oxygen-O
Iron- Fe	Germanium - Ge	Chlorine –Cl
Mercury Hg	Antimony -Sb	Nitrogen – N
Cadmium Cd	Arsenic - Sb	Carbon – C
Sodium Na	Tellurium -Te	Sulfur – S
Calcium Ca		Phosphorus -P
Chromium Cr		Bromine -Br

Is Gold a Metal Nonmetal or Metalloid–

Gold, Au, is one of the transition elements. It has an atomic number of 79 and is placed in group 11 of the table. It is a transition metal.

Is Sodium a Metal Nonmetal or Metalloid–

Sodium is placed in the first group of the periodic table. It is a soft metal. Sodium is one of the alkali metals.

Is Calcium a Metal Nonmetal or Metalloid-

Calcium, a white amorphous solid, is a metal, because of its electron donating property. Calcium is an alkaline earth metal.

Ionic Compound

Elements combine together to form compounds. Chemical compounds are of many types, depending upon the bond present in them. Corresponding to the two ways by which any two atoms rearrange to form a compound, two types of bonds are formed.

1. Ionic bond
2. Covalent bond.

Ionic bond or electrovalent bond is established by the transfer of one or more valence electrons from one atom to the other.

Thus, ionic bond is a chemical bond formed between two atoms by the transfer of one or more valence electrons from one atom to the other.

This bond is also called as a polar bond.

Formation of an ionic bond:

Formation of an ionic bond can be explained using the following example:

Consider an atom A , which has two electrons in its outermost shell. Another element B, has 6 electrons in its outermost shell.

The atom A has two electrons in excess, to make it to the Noble gas electronic configuration, while atom B has two electrons less to make it to that level.

Now, atom A gives two of the excess electron to atom B, and by this, atom A attains a completely filled shell, while atom B, having attained the required amount of electrons, also reaches the noble gas configuration.

They therefore form an ionic bond between themselves.

List of Ionic Compound Formula-

Lists of ionic-compounds with their formula are:

S.No	Ionic-compound	Formula
	Sodium chloride	NaCl
2.	Potassium iodide	KI
3.	Lithium iodide	LiI
4.	Aluminium oxide	Al2O3

Ionic Compound Example-

Some examples of ionic compound are:

Magnesium oxide - MgO – Mg²⁺, O^2-

Calcium Fluoride – CaF₂ – Ca²⁺, 2F^-

Aluminium Fluoride – AlF₃ – Al³⁺, F^3-

Properties of an Ionic Compound-

1. Ionic-compounds are three dimensional solids, with well-defined geometrical pattern.
2. Ionic solids conduct electricity when they are in water solution or in the fused state (molten state).
3. They are quite hard, have low volatility and high melting and boiling point.
4. Ionic solids are soluble in polar solvents, due to dissociation of their ions.
5. Ionic solids are very stable and have very high density.

Is NaCl an Ionic Compound-

Sodium chloride, NaCl is ionic in nature.

The formation of sodium chloride is as follows:

Na has an electronic configuration of 2, 8, 1. The last electron, in the third shell, has to be removed, for it to attain noble gas configuration.

Chlorine has a configuration of 2, 8, 7. It needs one extra electron, which it gains from Sodium, thereby getting the required magic number of ‘8’.

Is Salt an Ionic Compound–

Sodium chloride is also called as common salt. Other than this, most of the compounds, commonly known as salts are formed from the neutralization reaction of an acid and a base. They are all ionic-compounds, because they dissociate into ions in their solution.

Bonding and molecular structure

Syllabus : 

Valency electrons, the octet rule. Electrovalent and covalent bonds with examples. Properties of electrovalent and covalent compounds. Limitation of octet rule (examples), coordinate covalent bonds (examples).

Directionality of covalent bonds, shapes of polyatomic molecules (examples), concept of hybridization of atomic orbitals (qualitative pictorial approach): sp³, sp² and sp hybridizations with typical examples. Tetrahedral space model of-carbon atom, single-bond, double- bond and triple - bond involving carbon atom with examples a and 7t bonds.

Valence shell Electron Pair Repulsion (VSEPR) concept (elementary idea) - shapes of H₂0, H₂S,
CH₄, NH₃, C0₂, N0₂ and S0₂ molecules. Concept of resonance (elementary idea), resonance structures (examples). Elementary idea about electronegativity, bond polarity and dipole moment. Hydrogen bonding (inter - & intra molecular structures) and its effects on physical properties (mp, gp, and solubility).

Double salts and complex salts, and coordination compounds (examples only), coordination number (examples with C.N 4 and 6 only).

Valency Electrons and the Octet rule

When details of the electronic configurations of the elements came to be known, it was found tha the arrangement of energy levels in different orbits round the nucleus was different for different elements. Chemical union between atoms to form molecules are energetically favoured only if the chemical combination leads to lowering of energy of the system i.e, if the energy of the combined atoms i.e., the product molecule is less than the sum of the energies of the reactant molecules.

Chemical combination of atoms involves electrons of the two atoms and nuclei take no active part in chemical combination. In each individual atom, the outermost electrons have the highest energy amongst all its electrons. So the lowering of energy must be through the interactions of the outermost electrons of the two interacting atoms. Thus chemical combination in all probabilities, should involve only the outermost electrons of the participating atoms and the interactions should be such that it causes lowering of energy w.r.t. to the initial condition when the atoms lay separated from each other.

A close study of chemical properties of the different classes of elements led us to the fact that the noble gases were the most chemically inactive species amongest all the elements. They had no tendency to combine with themselves or with other elements. They were so inert that they even did not like to form molecules by the combination of two atoms. Inert gas molecules are monatomic — or in other words their atoms do not form molecules. They are devoid of any chemical affinity. mSo it may be seen in the light of our discussion above that their outer electronic configurations are most stable amongest all the elements and no further stabilization is possible by further interaction among themselves or with electrons in other elements.

The extra nuclear electrons present in n successive inert gases are 2,10,18, 36,54 and 86. These numbers were termed magic numbers as the presence of electrons in any one of these numbers in an atom gives special stability to the atom. This stability is lost if this number is changed even by 1 unit on either side.

The arrangement of the electrons in the inert gases can be described as follows :
He — 2
Ne — 2-8
Ar — 2-8-8
Kr —2-8-18-8
Ne —2-8-18-18-8

Molecular mass of polymers

Introduction :  
Several simple organic molecules of one or two types combine with each other by chemical bonds forming macro molecules the product is called a polymer. Simple organic molecules which can form polymers by chemical bonding are called monomers while this process of chemical combination is called polymerisation. In any sample or monomers there are very large number of molecules of lower masses same molecular weights and similar physical and chemical properties.

Polymers:

Ehereas in polymer sample having same molecular weights the number of molecules are very small. The polymers have comparatively very high molecular weight but all molecules do not have comparatively very high molecular weight but all molecules do not have identical molecular weights. Polymers prepared from the same monomer in different conditions do not have all the properties identical. Depending on reaction conditions polymer products have different proportions of molecules of lower and higher masses.

Molecular mass of Polymers:
Overall molecules of ethene combine with each other by addition reaction and give polyethene. This reaction at the first stage two molecules of ethene monomer combine together giving a dimer. A third molecule of ethene combines with this dimer giving a trimer and a forth molecule combine further gives a tetramer. This way monomer molecules go on joining and chain becomes longer. As result very large chain is former which is called macro molecule or polymer.
CH₂ = CH₂ `stackrel(CH_2=CH_2)(->)` CH₃-CH₂-CH = CH₂ `stackrel(CH_2=CH_2)(->)`
CH₃-CH₂-CH₂-CH₂-CH=CH₂ `stackrel(CH_2=CH_2)(->)`
CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH=CH₂
`stackrel(nCH_2=CH_2)(->)`  [-CH₂-CH₂-]_n

The polymer chain can be lengthen upto certain limit at laboratory cantons. The tendency of this long chain then decrease to combine with further monomers. Thus in any condition polymers resulting from monomers do not increase in weight more than a certain limit. Generally any polymer asmple contains varying chain-lengths, its molecular mass is always an average molecular mass. The molecular mass of a polymer is expressed as number average molecular mass `barM_n` or weight average molecular mass `barM_w`.
`barM_n = (sumN_tM_t)/(sum tN_t)`
`barM_w = (sumN_tM^2_t)/(sumtN_tM_t)`
Where N_t = number of molecules
M_t = molecular mass.

Molecular Orbital Theory

Introduction :
Molecular orbital theory is a polycentric region in space, defined by its size and shape, associated with two or more atoms in a molecule and each has a capacity of two electrons with opposite spins. Thus, in a molecular orbital, electrons are revolving in the field of more than one nucleus. The molecular orbital to explain formation of chemical bond, relative bond strengths, paramagnetic or diamagnetic nature.

Feature of Molecular Orbital Theory:

• The main features of the Molecular Orbital Theory are:
• The atomic orbital of the combine atoms partly cover to form new orbital, called molecular orbital.
• As a result of this, the atomic orbitals lose their being identity.
• Thus in a Molecular Orbital Theory, electrons revolves in the field of more than one nucleus.
• The number of Molecular Orbital is produced is equal to the number of overlap atomic orbitals.
• Maximum capacity of a Molecular Orbital is two electrons with opposite spins.
• Only those atomic orbitals can come together to form Molecular Orbital Theory which has analogous energies as well as proper orientations.
• Molecular orbital theory obtained addition of wave functions of atoms involved,`Psi`(MO)=`Psi`_A + `Psi` _B   is called bonding molecular orbital.
• Molecular orbital theory obtained by subtraction of wave functions of atoms involved  `Psi`  ^*(MO)=`Psi`_A - `Psi`_B is called antibonding molecular orbital.
• Probability of bonding molecular orbital formation greater than that of antibonding molecular orbital formation.
• Molecular theory gives the electron probability distribution around a group of nuclei just gives the electron probability distribution around nucleus.
• The shape of the molecular orbital theory produced depends on the type of the combining atomic orbitals.
• Inner molecular orbital theories which do not take part in bond formation are called non-bonding molecular orbital theory.

 Conditions for the formation of molecular orbitals:

• Any two atomic on combination do not form molecular orbitals.

• In fact, there are certain limitations to the combination of atomic orbitals.

• The energies of combining atomic orbitals should of similar magnitude.

• Thus, a homonuclear diatomic molecule will not be formed. if 1s orbital of one atom overlaps with 2s-orbital of another atom.

• Combination of atomic orbitals takes place only, if overlapping takes place to a considerable extent, since greater the overlapping of atomic orbitals, the greater is the build-up of the charge between the nuclei.

• The combining atomic orbitals should have power over the same symmetry about the molecular axis.

Chemical equilibrium animations

Introduction :

The experimental observations of chemical equilibrium tell us that most of the chemical reactions when carried out in closed vessels do not go to completion. Under these a conditions, a reaction starts by itself or by initiation, continues for some time at diminishing rates and ultimately appears to stop. The reactants may still be present but they do not appear to change into products any more. What happens in such case is that the products of the reaction start reacting at the same rate as the reactants. In other words, the rate of the back reaction becomes equal to the rate of the forward reaction.

Characteristic features of chemical equilibrium

Thus, in a given time as much of the products are formed as react back to give the reactants. The composition of the reaction mixture at a given temperature is the same irrespective of the initial state of the system, i.e., irrespective of the fact whether we start with the reactants or the products. The reaction in such conditions is said to be in a state of equilibrium.

The attainment of equilibrium can be recognized by noting constancy of observable properties such as pressure, concentration, density or color whichever may be suitable in a given case.

The relationship between the quantities of the reacting substances and the products formed can be worked out readily with the help of the law of mass action.

The Laws of Mass Action of Chemical equilibrium

The laws of mass action states that the driving force of a chemical reaction is proportional to the active masses of the reacting substances. Assuming that the driving force determines the reaction rate, the law may be stated as follows: The rate at which a substance reacts is proportional to its active mass and the rate of the chemical reaction is directly proportional to the product of the active masses of the reacting substance.

Consider a general reversible chemical reaction

aA + bB ↔ mM + nN

According to the law of mass action, assuming that active masses are equivalent to molar concentrations,

The rate of the forward reaction, r_f α [A]^a [B]^b = K_f[A]^a [B]^b

The rate of the reverse reaction, r_r α [M]^m[N]ⁿ = K_r[M]^m [N]ⁿ

Where Kf and Kr are proportionally constants and square brackets represent the molar concentrations of the entities enclosed. At equilibrium, the rate of the frontward reaction is equal to the rate of the reverse reaction, that is, Kf[A]a [B]b = Kr[M]m [N]n

K_f /K_r = K_eq = [M]^m [N]ⁿ / [A]^a [B]^b