Class 11 Some Basic Concepts Of Chemistry Note's
Some Basic Concepts Of Chemistry
# ELEMENTS :
- Elements is the purest form of matter . It is made up of only one type of atom .
MIXTURE
- Mixture is the substance which is made up of two or more substances in any ratio.
e.g. Sugar+ Water, Sodium Chloride + Water, Sand+ Water
On the basis of composition, mixtures are of following type:
1. Homogeneous mixture: The mixture which has uniform composition through out
e.g., sugar solution.
2. Heterogeneous mixture: The mixtures which do not have uniform composition through out.
e.g. sand in water.
ATOM
Atom is the smallest particle which may or may not exist free but takes part in chemical reaction. Atom word means not be cut .
Significant Figures
How does one express the uncertainty about accuracy of measurement while writing the result of a ovation? The total number of digits is called the number of significant figures. It represents the accuracy and precision with which a quantity has been measured.
Remember : The number of significant figures is the number of digits in result including the last digit which is uncertain .
Rules :
1) All digits are significant except zero in the beginning of a number
2) Zeros to the left of the first non-zero digit are not significant if such zeros follow the decimal point
3) Zeros to the right of the decimal point are significant
4) Counting the numbers of object, for example, 2 balls or 20 eggs, have infinite significant figures as these are exact numbers and can be represented by writing infinite number of zeroes after placing a decimal i.e 2 = 2.000000 or 20= 20.000000
Laws of Chemical Combination
1. Law of conservation of mass
2 Law of constant composition
3. Law of multiple proportions
4 Law of reciprocal proportions
5) Gay Lussac's law of combining volumes
1) Law of conservation of mass or law of undestructiability: It states that the total mass is conserved during a chemical reaction.
Total mass of reactants = total mass of products + mass of unreacted reactant (if any)
2. Law of Constant Composition or Definite Proportions: This law states that a chemical compound is always found to be made of same elements combined together in fixed proportion by weight.
CO₂ can be prepared by number of methods but always 12 g carbon react with 32 g of oxygen.
3) Law of Multiple Proportions : When two elements combine to form two or more chemicals compounds , then weight of one of the elements which combines with a fixed weight of the other , bears a simple whole number ration to one another . This is called the law of Multiple Proportions .
4) Law of Reciprocal Proportions : It states that the ratio of weight of two elements A and B , which combine separately with the fixed weight of a third elements C is either same or some simple whole number if the ratio of weight in which A and B combine directly with each other .
5) Gay Lussac's Law of combination volumes : According to this law , when gases react together , they always do so in volume which bear a simple ratio to one another and to the volume of products , if these are also gases .
Dalton's Atomic Theory :
By observing the laws of chemical combination as already discussed, John Dalton in 1808 published "A New System of Chemical Philosophy" in which he proposed atomic theory of matter. The main points of Dalton's atomic theory are as follows :
(1) Matter is made up of extremely small, indivisible particles called atoms.
(2) Atoms of a given element are identical in all respect, Le, they possess same size, shape, mass, chemical properties etc.
(3) Atoms of different elements are different in all respects, Le they possess different sizes, shapes, masses chemical properties etc.
(4) Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compounds.
(5) Atoms can neither be created nor destroyed in a chemical reaction
Dalton's theory could explain the laws of chemical combination
ATOMIC WEIGHT AND MOLECULAR WEIGHT :
- Atomic weight = mass of one atom / "1/12 of mass of one atom of C12 "
- Molecular weight = Mass of one molecule / "1/12 of mass of one atom of C12 "
## 1Amu = 1.66 X 10-24 g
Atomic mass and molecular Mass
Atomic mass = mass of one atom X "1/12 of mass of one atom of C12 " / "1/12 of mass of one atom of C12 "
Average Atomic Mass : Elements are found in different isotopic form , so the atomic mass of any elements is the average of all the isotopic masses a given sample .
Average Molecular weight = Σ (% abundance X molecular wt. of constituents ) / 100
THE MOLE :
One Mole : one mole is the amount of substance that contains as many as entities as number of atoms in exactly 12.00 g of C12 .
Number of carbon atoms in 12 g C12 = 6.022 X 1023
1) 1 mol = 6.022 X 1023
2) 1 mol = 22.4 litre of gas at STP
IMPORTANT FORMULA :
n = Weight / Molar mass
n = Number of molecules of entites / Avogadro Number
n = Volume of gas at STP / 22.4 litre
MOLAR MASS :
The mass of one mole of a substance in grams is called its molar mass. The molar mass in grams is numerically equal to atomic, moleculer mass in u.
Example: Molar mass of ammonia = 17.0g
Gram atomic mass or gram atom: Atomic weight of an element in grams is called gram atomic mass of an element. Gram atomic mass is the weight of 6.022 x 1023 atoms of carbon = 1 mole atom of C = 1 g atom of carbon.
Molecular mass: It is defined as the mass of one mole of a substance formed by a combination of one or more elements. Molecular mass expressed in grams is known as gram molecular mass.
Percentage of an elements in compound :
% by weight = Wt. of element X 100 / mol . Wt. of compound
Empirical Formula (EF)
The formula which expresses the smallest whole number ratio of the constituent atoms within the molecule Empirical formula of different compounds may be same. So it may or may not represent constituent elements with their atomic masses for a given compound.
For example: CH12O6, CH3COOH, HCHO.
All compounds have same empirical formula CH₂O.
Molecular Formula (MF)
The formula which represents the actual number of each individual atom in any molecule is known as molecular formula.
For certain compounds the M.F. and E.F. may be same.
Molecular formula = (Empirical formula)n
Molecular formula weight = Empirical formula weight x n
If the vapour density of substance is known, its molecular weight can be calculated by using the equation 2 × Vapour density = Molecular weight.
Relationship between Empirical and Molecular Formula
Molecular formula = [Empirical formula]n
or, n= Molecular mass / Empirical formula mass
SOLUTION :
Solution is the homogeneous mixture of two or more than two substance . Substance of the solution are classified into Components
i) SOLUTE
ii) SOLVENT
Solvent : Solvent is the substance which has same physical state as of the function .
MEASUREMENT OF CONCENTRATION
The concentration of a solution reflects the relative proportion of solute and solvent present in the solution .
The various concentration terms are :
1). % w/w (weight percent or Mass percent)
x %w/w means that x g solute is present in 100 g of solution.
% w/w = Weight of solute X 100 / Weight of solution
2). % w/V --- x % w/v means that x g of solute is present in 100 ml solution Weight of solute
% w/V- Weight of solute x100 / Volume of solution
3). % Vv - x % V/V means that x ml of solute is present in 100 m solution
% V/V = Volume of solute X 100 / Volume of solution
4). Molality (m) - It is defined as number of moles of solute present in 1 kg of solvent.
m = Number of moles of solute / Mass of solvent (in kg)
5) Molarity (M)-It is defined as number of moles of solute present in 1 L of solution .
M = Number of moles of solute / Volume of solution (in litre)
Molarity of the Mixture of Two Solutions of Same Solute and Solvent
M = M1V1 + M2V2 / V1
+ V2
Here, M1 and V1 are the molarity and volume of one solution and M2 and V2 are the molarity and Volume other solution
MOLE FRACTION : Suppose , n is the moles of solute and N is the moles of solvent , then ,
Mole fraction of Solute( Xsolute) = n / n +N
Mole fraction of solvent ( Xsolvent) = N / n +N
STOICHIOMETRY AND STOICHIOMETRIC CALCULATIONS
Calculations based on the quantitative relationship between the reactants and the products are referred as stoichiometry.
The term stoichiometry is derived from two Greek words - Stoicheion (meaning - element) and Metron (meaning-measure).
In order to solve the problems based on chemical calculations the following steps, in general, are quite helpful
(1) Write the balanced chemical equation
(ii) From the chemical equation write the molar ratio between the given and required substance
(iii) Convert these moles into the desired parameters such as mass or volume, like
Density = Mass / Volume
(iv) Calculate the final result by applying unitary method. (Unitary method consists of altering one of the variable to a single unit, ie., 1 and then performing the operation necessary to alter it to the desired value)