tag:blogger.com,1999:blog-31365074426378534462024-03-14T00:03:58.862-07:00Some useful facts of physical chemistryRavi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.comBlogger14125tag:blogger.com,1999:blog-3136507442637853446.post-55235686103357031992009-02-10T17:40:00.000-08:002009-02-10T17:53:14.259-08:00Radioative Decay<span style="font-size:130%;">Types of <span class="blsp-spelling-corrected" id="SPELLING_ERROR_0">Radioactive</span> Decay</span><br />The type of radioactive decay that occurs usually depends on whether the nucleus is above or <span class="blsp-spelling-corrected" id="SPELLING_ERROR_1">below</span> or to the right of the stability belt.There are five common types of radioactive decay.<br />1.Alpha <span class="blsp-spelling-corrected" id="SPELLING_ERROR_2">Emission</span><br />2.Beta <span class="blsp-spelling-corrected" id="SPELLING_ERROR_3">Emission</span><br />3.Positron Emission<br />4.Electron Capture<br />5.Gamma Emission<br /><span style="font-size:130%;">Half-life period</span><br />The time required for half of a radioactive substance to disintegrate is known as the half-life period, represented here by t1/2<br /><span style="font-size:130%;">Radioactive Isotopes and Isobars</span><br />A radioactive isotope is formed by the emission of one alpha and <span class="blsp-spelling-error" id="SPELLING_ERROR_4">bita paricles</span>, while an isobar is formed by the emission of one <span class="blsp-spelling-error" id="SPELLING_ERROR_5">bita</span> particle.<br /><span style="font-size:130%;">Binding Energy of the Nucleus</span><br /> It has been observed that the actual mass of an isotope of an element, obtained experimentally using mass spectrograph, is invariably less than the calculated mass of the isotope. This mass difference is known as mass defect of the nucleus which is also expressed as the packing fraction of the nucleus.<br />Packing fraction = isotopic mass - mass number/mass number x 10000Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-51082789054029961272009-02-05T19:36:00.000-08:002009-02-05T19:43:16.879-08:00Neutron-Proton Ratio and Nuclear StabilityMost naturally occurring nuclides hav even numbers of neutrons and even numbers of protons. Nuclides with odd numbers of both neutrons and protons are least common and those with odd-even combinations are intermediate in abundance.<br />No. of protons even even odd odd<br />No. of neutrons even even even odd<br />No. of stable nuclides 157 52 50 5<br />A plot of the number of neutrons versus the number of protons shws that as the atomic number increases, the N/P ratio of the stable nuclides increases.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-45289561918604347752009-02-04T06:05:00.000-08:002009-02-04T06:17:19.203-08:00Quantum Numbers and Electronic ConfigurationThe following rules help us to predict electronic configuration<br />of atoms with the help of quantum numbers<br />1. Electrons are added into orbitals in the way that gives the<br />lowest total energy for the atom.<br />2. Electrons are assigned to orbitals in order of increasing<br />value of (n+l).<br />3. For subshells with the same value of (n+l), electrons are<br />assigned first to the subshell with lower n.<br />4. No two electrons in an atom may have identical sets of four<br />quantum numbers.<br />5. Electrons occupy all the orbitals of a given subshell singly<br />before pairing begins. These unpaired electrons have parallel<br />spins. (Hund Rule). Hund rule states that for degenerate<br />orbitals, the lowest energy is attained when the number of<br />electrons with the same spin is maximized.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com1tag:blogger.com,1999:blog-3136507442637853446.post-14761080909065073252009-01-30T19:25:00.000-08:002009-01-30T19:28:08.602-08:00Atomic StuctureAn atom consists of <span class="blsp-spelling-corrected" id="SPELLING_ERROR_0">extremely</span> small and dense nucleus and an <span class="blsp-spelling-corrected" id="SPELLING_ERROR_1">extra nuclear</span> space. The nucleus contains positively charged protons and neutrons, and these particles are collectively called <span class="blsp-spelling-corrected" id="SPELLING_ERROR_2">nucleons</span>. In the <span class="blsp-spelling-corrected" id="SPELLING_ERROR_3">extra nuclear</span> space, negatively charged electrons revolve around the nucleus. As the magnitude of the charge of an electron is the same as that of a proton, the number of electrons is equal to that of protons in an atom, the atom being neutral.The number of protons present in the nucleus of an atom is termed as the atomic number of the element (Z). The sum of the number of protons and neutrons is called the mass number (A). The <span class="blsp-spelling-corrected" id="SPELLING_ERROR_4">term</span> '<span class="blsp-spelling-error" id="SPELLING_ERROR_5">nuclide</span>' refers to a nucleus having a specific atomic number and specific mass number.To calculate the radius (r) and energy (E) of a permissible orbit for one electron species Bohr derived equations based on the following postulates.<br />Bohr's Postulates<br />1. The electrons revolve around the nucleus in certain orbits without losing energy because the energy in a fraction of a quantum can neither be lost nor gained.<br />2 Energy is absorbed or emitted only when an electron in an atom jumps from one orbit to another.<br />3 <span class="blsp-spelling-corrected" id="SPELLING_ERROR_6">The</span> electron is restricted to those orbits in which its angular momentum is an integral multiple of h/2<span class="blsp-spelling-corrected" id="SPELLING_ERROR_7">pie Angular</span> momentum = <span class="blsp-spelling-error" id="SPELLING_ERROR_8">mvr</span>= n x h/2pie<br />Failure of Bohr Theory<br />1 It does not explain the spectra of species having more than one electron.<br />2 It does not explain the fine spectral lines obtained under a spectroscope of strong resolution. However, it can be explained by Bohr-<span class="blsp-spelling-error" id="SPELLING_ERROR_9">Sommerfeld</span> theory of elliptical orbits.<br />3 It does not explain <span class="blsp-spelling-error" id="SPELLING_ERROR_10">Zeeman</span> effect, that is, splitting of spectral lines under magnetic field, and Stark effect, that is, splitting of spectral lines under electric field.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-51415907252191078432009-01-27T17:17:00.000-08:002009-01-27T17:21:47.532-08:00Empirical, Molecular and Structural FormalaeThe utility of the mole concept is further illustrated by the<br />problems of determining the empirical and molecular formulae of<br />the compounds. Empirical formula represents the simplest set of<br />whole numbers expressing the relative numbers of atoms in the<br />compound and anything that can be said about relative numbers of<br />atoms may be said about the relative numbers of moles of atoms. A<br />calculation of the relative numbers of moles of each element in<br />the compound will, <span class="blsp-spelling-corrected" id="SPELLING_ERROR_0">therefore</span>, lead us to the empirical formula of<br />the compound. The empirical formula implies nothing about how<br />many moles of atoms are actually in one mole of the compound.In fact, the molecular formula expresses the actual numbers of moles of atoms of each element present in one mole of the compound. The molecular formula weight is the whole <span class="blsp-spelling-corrected" id="SPELLING_ERROR_1">number</span> multiple of the empirical formula weight for a given compound.Molecular formula weight/Empirical formula weight = n(say)Thus if X <span class="blsp-spelling-corrected" id="SPELLING_ERROR_2">represents</span> the empirical formula of a compound, <span class="blsp-spelling-corrected" id="SPELLING_ERROR_3">its</span> molecular formula will be represented as (X)n.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-956620761511721512009-01-26T17:53:00.000-08:002009-01-26T17:56:05.377-08:00Electrolytic ConcuctanceThe passage of a current through an electrolyte involves the movement of ions carrying an electric charge and so the study of electrolytic conduction may supply useful chemical information.The magnitude of the conductance, i.e., the reciprocal of resistance, depends mainly on three factors: the number of ions, magnitude of charge on each ion and the ionic mobility.The conductance of an <span class="blsp-spelling-corrected" id="SPELLING_ERROR_0">electrolyte</span> may be measured in terms of molar conductance, that is, the conductance due to one mole of an ionic solute and secondly, the equivalent conductance, that is, the conducting power of all ions produced by one equivalent of the electrolyte in the given solution. But to compare the conductance of two solutions, equivalent <span class="blsp-spelling-corrected" id="SPELLING_ERROR_1">conductance</span> is considered because one equivalent of different electrolytes involves the same number of electrons in accordance with Faraday's second law of electrolysis while one mole of different electrolytes may or may not involve the same number of electrons. In other words, the solutions, each containing one equivalent of different electrolytes, are equivalent in terms of moles of electrons being carried.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-44742027588535888802009-01-25T19:14:00.000-08:002009-01-25T19:25:07.183-08:00Importance of Faraday's lawThe electrical and chemical concepts are interdependent. A <span class="blsp-spelling-error" id="SPELLING_ERROR_0">flow</span> of electricity through a substance may produce a chemical reaction, and also, a chemical reaction may cause a <span class="blsp-spelling-error" id="SPELLING_ERROR_1">flow</span> of electricity through some external circuit. The former involves the study of electrolysis and conductance, while the latter, the measurement of electromotive force.<br /><span style="font-size:130%;">Electrolysis<br /></span>Faraday's <span class="blsp-spelling-corrected" id="SPELLING_ERROR_2">law The</span> <span class="blsp-spelling-corrected" id="SPELLING_ERROR_3">quantitative relationship</span> between the amount of electricity passed through a cell and the amount of substances <span class="blsp-spelling-corrected" id="SPELLING_ERROR_4">discharged</span> at the electrodes was systematised by Michael Faraday in the form of the following laws:<br /><span style="font-size:180%;"><strong>First law</strong></span> : The amount of substance discharged at an electrode is proportional to the quantity of the <span class="blsp-spelling-corrected" id="SPELLING_ERROR_5">electricity passing</span> through the electrolyte.<br /><span style="font-size:180%;"><strong>Second law</strong></span>: When the same quantity of electricity is passed through <span class="blsp-spelling-corrected" id="SPELLING_ERROR_6">different</span> solutions, the amount of different substances deposited or dissolved at the electrodes in different electrolytic cells are proportional to their equivalent weights, and in an electrolytic cell, chemically equivalent amounts or substances are <span class="blsp-spelling-corrected" id="SPELLING_ERROR_7">discharged</span> at both the electrodes.<br /><em><strong>The essential content of Faraday's second law is that 1 <span class="blsp-spelling-corrected" id="SPELLING_ERROR_8">Faraday</span>, which corresponds to 1 mole of electrons, liberates 1 <span class="blsp-spelling-corrected" id="SPELLING_ERROR_9">equivalent</span> of matter. In <span class="blsp-spelling-corrected" id="SPELLING_ERROR_10">redox</span> reactions, the amount of the reactant, corresponding in 1 mole of electrons, is thus its equivalent mass.</strong></em>Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-34618095330601130172009-01-23T19:29:00.000-08:002009-01-23T19:39:11.495-08:00Chemical EequivalenceSignificance of Equivalent WeightAn equivalent of a substance is defined as the amount o it which combines with 1 mole of hydrogen atoms or replaces the same number of hydrogen atoms in a chemical reaction. The weight in grams of 1 equivalent is called the equivalent weight in grams. For example, in the compounds HBr one mole of H combines with one mole of Br, 1/2 mole of O and 1/3 mole of N respectively. Hence the equivalent weights in grams of Br, O and N are the weights of 1 mole each of Br, O and N contains their 1 equivalent, 2 equivalents and 3 equivalents respectively.Thus : eq.wt. of Br= 1x79.9=79.9<br />To determine the equivalent weight of an element, it is not necessary to proceed from its hydrogen compound only. Eequivalent weight of an element can be calculated using the composition of the compound of the given element with any other element, whose equivalent weight is known by the knowledge of the Law of Equivalence. The law states that one equivalent of an element combines with one equivalent of the other. Accordingly, the equivalent weight of an element is the weight of its moloe combining with one equivalent of another element.The equivalent weight of a compound taking part in a reaction is the weight of the compound which combines with 1 equivalent of another compound. Thus knowing that 1 mole of HCl is equal to 1 equivalent of it, equivalent weight of Sodium carbonate can be calculated from the the equation. In an acid-base neutralisation reaction the equivalent weight of an acid is that portion of the weight of 1 mole of the acid which can furnish 1 mole of H, and the equivalent weight of a base is the portion of weight of one mole of the base which can furnish 1 mole of OH or accept 1 mole of H. In redox reactions, the equivalent weight of an oxidising or a reducing agent is the portion of the weight of 1 mole of substance that picks up or releases 1 mole of electrons respectively.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-37229583080926489722009-01-20T14:15:00.000-08:002009-01-20T14:35:20.363-08:00Molecular WeightMolecular weight of a compound is defined as the weight of a molecule of the compound relative to a carbon atom, the atomic weight of which is supposed to be exactly 12. The molecular weight when expressed in grams is called gram molecular weight. The molecular weight in grams is, in fact, the weight of 1 mole of molecules, e.g., molecular weight of oxygen is 3 and 32 g is the weight of 1 mole of oxygen molecules. Mathematically,<br /><br /><strong><em>molecular weight = weight of molecules in grams/number of moles of molecules</em></strong><br /><br />Molecular weight is measured in atomic mass unit (<span class="blsp-spelling-error" id="SPELLING_ERROR_0">amu</span>). Atomic mass unit is defined as 1/12 of the mass of the carbon isotope. There are various methods to determine the molecular weight of compounds viz,. vapour-density method. depression-in-freezing-point method, elevation-in-boiling-point method, <span class="blsp-spelling-error" id="SPELLING_ERROR_1">gravimetric</span> method, volumetric method, etc. The problems on molecular weight based on depression-in-freezing-point and elevation-in-boiling-point methods shall be discussed in Next blogs.<br />The mole method is found to be very useful in tackling the problems on molecular weight based on the aforesaid methods.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-63212671303847523742009-01-18T18:32:00.000-08:002009-01-18T18:40:45.131-08:00Some about Atomic weight<strong><span style="font-size:130%;">Atomic <span class="blsp-spelling-corrected" id="SPELLING_ERROR_0">weight</span></span></strong><br /><span class="blsp-spelling-corrected">The</span> atomic weight of an element is defined as the average weight of the atoms of the element relative to a carbon atom, taken as exactly 12. Atomic weight in grams is, in fact, the weight of one mole of atoms, e.g., the atomic weight of oxygen is 16 and so 16 grams is the weight of 1 mole of oxygen atoms.<br /><strong>Mathematically,</strong><br />atomic weight = weight of atoms in grams/number of moles of atoms<br />Atomic weight is measured in atomic mass unit. Atomic mass unit is defined as 1/12 of the mass of the C isotope. One <span class="blsp-spelling-error" id="SPELLING_ERROR_1"><strong><em>amu</em></strong></span> is also called one <strong><em><span class="blsp-spelling-error" id="SPELLING_ERROR_2">dalton</span>.</em></strong>Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-45682365108852246632009-01-16T18:29:00.000-08:002009-01-16T18:38:50.630-08:00Gas Analysis<strong><span style="font-size:130%;"><span style="color:#3333ff;">Gas Analysis</span><br /></span></strong>Gaseous reactions are carried out in a special <span class="blsp-spelling-corrected" id="SPELLING_ERROR_0">type</span> of tube known as an <span class="blsp-spelling-error" id="SPELLING_ERROR_1">eudiometer</span> tube. The tube is graduated in millimetres for volume measurement. The reacting gases taken in the <span class="blsp-spelling-error" id="SPELLING_ERROR_2">eudiometer</span> tube are exploded by sparks, produced by passing electricity through the platinum terminals provided in the tube. The volumes of the products of a gaseous explosion are determined by absorbing them in suitable reagents, e.g., Carbon dioxide and sulphur dioxide are absorbed in <span class="blsp-spelling-error" id="SPELLING_ERROR_3">KOH</span> solution, Oxygen is absorbed in a solution of alkaline <span class="blsp-spelling-error" id="SPELLING_ERROR_4">pyrogallol</span>, Water vapour produced during the reaction changes to liquid on cooling, the volume of water is neglected, but while applying <span class="blsp-spelling-error" id="SPELLING_ERROR_5">POAC</span>, moles of Water produced cannot be neglected.<br /><span class="blsp-spelling-error" id="SPELLING_ERROR_6">Eudiometry</span> is mainly based on Avogadro.s law, which states that equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules. Two gases having equal number of applied in solving the problems of this chapter, keeping in mind that in a gaseous reaction the relative volumes of each reactant and product represent their relative numbers of moles.Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-78874742964964249512009-01-15T17:52:00.000-08:002009-01-15T17:59:36.059-08:00About Stoichiometry<strong><span style="font-size:130%;">STOICHIOMETRY</span></strong><br />The word stoichiometry is derived from the Greek words stoicheion, which means element, and metrein, which means to measure. The numerals used to balance a chemical equaton are known as stoichiometric coefficients. These numbers are essential for solving problems based on chemical equations. Hece such problems are also called stoichiometric calculations.<br />For stoichiometric calculations, the mole relationships between different reactants and products are required, as from them, the mass-mass, mass-volume and volume-volume relationships between different reactants and products can be obtained.<br /><strong><span style="font-size:130%;">Concept of Limiting Reagent</span></strong><br />In single-reactions, the calculations are carried out wi only that amount of teh reactant which has converted to the product. This is done in all the methods mentioned above.<br />In the reactions where more than one reactant is involved, one has to first identify the limiting reactant, i.e., the reactant which is completely consumed. All calculations are to be carried out with the amount of the limiting reactant only. Now the questioni s how to identify the limiting reactant? The procedure is simple as illustrated below:<br />Initially.... 5 moles 12 moles 0 moles<br /> A + 2B ----> 4C<br />If A is the limiting reactant : moles of C producec = 20<br />If B is the limiting reactant : moles of C produced = 24<br />The reactant producing the least number of moles of the product is the limiting reactant and hence A is the limiting reactant. Thus,<br />Initially .... 5 moles 12 moles 0 moles<br /> A + B -------> 4C<br />Finally.... 0 moles 2 moles 20 moles<br /><em>The limiting reactant can also be ascertained by knowing the initial number of equivalents of each reactant. The reactant with the least number of equivalents is the limiting reactant. The equivalent method to identify the limiting reactant can be applid even if the chemical equation is not balanced.<br /></em>Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-71388887345788666372009-01-13T07:10:00.000-08:002009-01-13T07:19:40.089-08:00Mole Concept<div align="left"><br /><span style="font-size:130%;">Mole Concept<br /></span>The mole concept is an essential tool for the study of the fundamentals of chemical calculations.<br />Mole in Latin means heap or mass or pile. A mole of atoms is a collection of atoms whose total weight is the number of grams equal to the atomic weight. As equal numbers of moles of different elements contain equal numbers of atoms, ti is convenient to express amounts of the elements of the elements in terms of moles. Just as a dozen means twelve objects, a score means twenty objects, chemists have defined a mole as a <span class="blsp-spelling-error" id="SPELLING_ERROR_0">definite</span> number of particles. This definite number is called the Avogadro constant, equal to 6.022 X 10 to the power 23, in honour of the concept of a mole without knowing the v<span class="blsp-spelling-error" id="SPELLING_ERROR_1">alue</span> of the <span class="blsp-spelling-corrected" id="SPELLING_ERROR_2">Avogadro</span> constant. Thus, a mole of hydrogen atoms or a mole of hydrogen molecules or a mole of hydrogen ions, or a mole of electrons means the <span class="blsp-spelling-corrected" id="SPELLING_ERROR_3">Avogadro</span> <span class="blsp-spelling-corrected" id="SPELLING_ERROR_4">constant</span> of hydrogen atoms, hydrogen molecules, hydrogen ions or electrons respectively.<br />The value of the Avogadro constant depends on the atomic-weight scale. The mole is defined as the amount of a substance containing as many atoms, molecules, ions, electrons or other elementary entities as there are carbon atoms in exactly 12 g of 12C.<br />In modern practice, a gram-molecule and a gram-atom are termed as a mole of molecules and a mole of atoms respectively,<br /><span style="color:#3333ff;"><span style="font-size:130%;">Significance of Chemical Equations</span><br /></span>A chemical equation describes the chemical process both qualitatively and quantitatively. The <span class="blsp-spelling-error" id="SPELLING_ERROR_5">stoichiometric</span> coefficients in the chemical equation give the quantitative information of the chemical process. These coefficients represent the relative number of molecules or moles of the reactants and products.<br /><span style="font-size:130%;color:#3333ff;">Principle of Atom Conservation<br /></span>The principle of conservation of mass, <span class="blsp-spelling-corrected" id="SPELLING_ERROR_6">expressed</span> in the concepts o atomic theory, means the conservation of atoms. And if atoms are conserved, moles of atoms shall also be conserved. This is known as the principle of atom conservation. This principle is in fact the basis of the mole concept.<br /><span style="color:#3333ff;"><span style="font-size:130%;">Advantages of the Mole Method over other Methods</span><br /></span>The advantages of the Mole Method are as under:<br /><em>1. Balancing of chemical equations is not required in the majority of problems as the method of balancing the chemical equation is based on the principle of atom conservation.<br />2.Number of reactions and their sequence, leading from reactants to products, need not be given.<br />3.It is a general method, applicable in solving many types of problems as may be seen in different chapters.</em></div>Ravi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0tag:blogger.com,1999:blog-3136507442637853446.post-66365109125154380922009-01-08T23:09:00.000-08:002009-01-08T23:33:22.977-08:00Intoduction of physical chemistryChemistry is just one of the subjects you have to study and, therefore, you have limited time for each subject. Keeping inmind your limited time and requirements.<br />Physical chemistry is a branch of chemiiiistry which deals with physical state of compounds, elements or substances. To learn Physical chemistry is very essential for all those students who intersted in chemistry and want to go in depth of chemistry.<br />Firstly we have to know that what is physical chemistry and why it is so useful for chemistry. Here are some useful topics of Physical chemistry which we have to must read during knowing about chemistry. Without Physical chemistry we can not learn good about chemistry.<br />In Physical chemistry firstly we should have good knowledge of mole concepts because it is root of Physical chemistry and we all knew that without root nothing could.<br />Here are some useful Topics of Physical chemistry which I describe in another sections:<br />1.The Mole Concept<br />2.Stoichiometry<br />3.Gas Analysis<br />4.Atomic Weight<br />5.Molecular Weight<br />6.Chemical Equivalenece<br />7.Volumetric Calculations<br />8.Electrolysis<br />9.Estimation of Elements in Organic Compounds<br />10.Empirical, Molecular and Structural Rormulae<br />11.Atomic Structure and Radioactivity<br />12.Properties of Gases<br />13.Dilute Solution and Colligative Properties<br />14.Chemical Thermodynamics<br />15.Chemical Equilibrium<br />16.Ionic Equilibrium in Aqueous Solutions<br />17.CHemical Kinetics<br />18.Electromotive Force<br />19.Oxidation Number and Balancing of Redox Reaction<br />20. Solid and Liquid States<br />21.Solid and Liquid States<br />All above topics I described in stage by stage so read regularly and gain your knowledge.<br />GOOD LUCKRavi Shankerhttp://www.blogger.com/profile/13460705340660148075noreply@blogger.com0