Carbon is standard for the determination of the atomic masses of the other elements, its atomic mass is 12 a.m.u
(atomic mass unit)
12th part of the mass or carbon is 1 a.m.u and the masses of other elements are determined by the comparing with it.
For Example
Mass of hydrogen atom is equal to the 12th part of mass of carbon, thus the atomic mass of hydrogen is 1 a.m.u
Atomic mass of magnesium (Mg) is double than carbon, thus it is 24 a.m.u
Atomic masses of some elements are given below:
N = 14 a.m.u
O = 16 a.m.u
Na = 23 a.m.u
Ca = 40 a.m.u
Unit
There is no proper unit of atomic mass , its is expressed in atomic mass units (a.m.u).
Emperical Formula
Definition
Formula which shows tha simple ratio of the atoms in a compound is called emperical formula.
Example
1) Benzene
2) Glucose
3) Acetic acid
Emperical formula of common salt is NaCl and magnesium oxide is MgO
Generally ionic compounds are represented by the emperical formula because they have no molecules.
Molecular Formula
Definition
Formula which shows the actual number of atoms in a molecule of a substance is called molecular formula.
Or
Formula based on actual molecules is knows and molecular formula.
Example
Molecular formula of water is H2O which shows that the two atoms of hydrogen and one atom of oxygen are present in one molecule of water. Molecular formula of carbon dioxide is CO2 which shows that the one atom of carbon and the two atoms of oxygen are present in one molecule of carbon dioxide.
Generally covalent compound are represented by the molecular formula because these exist in molecules.
Formula Mass
Definition
Sum of atomic masses of all atoms in an emperical formula is called formula mass.
Mass of an ionic compound is also known as formula mass.
Like sodium chloride NaCl = 58.5 a.m.u
And magnesium oxide MgO = 40 a.m.u
Molecular Mass
Definition
Sum of atomic masses of all atoms present in a molecular formula is called molecular mass.
Example
Water = 18 a.m.u
Glucose = 180 a.m.u
Natural gas = 16 a.m.u
Mole And Avogadro's Number
Mole
Gram atomic mass, gram molecular mass, gram formula mass of a substance which contains 6.02*10^23 atoms, molecules or ions is known as mole.
It is determined by the Amedo Avogadro that if we take mass of substance in grams, it contains 6.02*10^23 particles of that substance , so the mass is known as 1 mole.
Avogadro's Number (NA)
Number of atoms, molecules or ions in one mole of a substance is called Avogadro's Number. It is denoted by NA. It was determined by Amedo Avogadro that one mole of a substance contain 6.02*10^23 particles of that substance. This huge number is called Avogadro's number.
Stoichiometry
Relationship between the amount of reactant and the product through a balanced chemical equation is called stoichiometry.
Example
Balanced equation for the formation of water 2H2 + O2-----> 2H2O. It shows that 2 mole or 4 grams of hydrogen combine with 1 mole of oxygen that is 32g to give 2 moles of water that is 36g of water
By the help of stoichiometry we calculate the amount of any reactant or product from the given compound of other through the balanced chemical equation.
Important condition for stoichiometric calculation:
There are two important conditions for the stoichiometric calculations.
1) All the reactant should change into product.
2) No side reaction take place, means no any other product is formed except the required product.
Stoichiometric Relationships:
There are three stoichiometric relationships.
1) Mass-Mass Relation:-
In this relationship we calculate mass of any reactant and product from the given mass of other by the help of balance chemical equation.
2) Mass-Volume Relationship:-
In this relationship we calculate the volume of any gaseous reactant or product from given mass of other by the help of balanced chemical equation.
According to the Avogadro's law 1 mole of any gas at STP (Standard temperature and pressure) 0 celsius and 1 atm occupy the 22.4 dm^3 volume.
3) Volume-Volume Relationship:-
In this relationship we calculate the volume of any reactant or product form
the given volume of other by the help of balanced chemical equation. This relationship is for the gasses.
Limiting Reactant
Definition
A reactant which is consumed first in the reaction is called limiting reactant.
Explanation
If one of the reactants in a reaction is in low amount than the required amount then it is consumed first and is called limiting reactant.
When the amount of the limiting reactant is consumed then the reaction becomes stop, and the amount of product formed is calculated from the amount of the limiting reactant.
Example
For the complete reaction 4 grams of H2 react with 32 grams of O2 to form 36 grams of H2O (2H2 + O2----->2H2O).
If we take 4 grams of H2 and 24 grams of O2 in a cylinder for the reaction then O2 is the limiting reactant because 24 grams of O2 consume first then the 4 g H2
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