Archive for October, 2008
As a high school student you may sometimes facing stoichiometry on your chemistry course. There are many students having difficulties on solving of stoichiometry problems. Do you know that it’s very easy to solve this problem if you recognize the technique. These simple methods will allow you to better understanding on the way how you solve the stoichiometry questions.
1. Just balance the equation of the reaction
Question about stoichiometry mostly involving chemical reaction, thus the important first thing you should do is balancing the equation. Check for the reaction coefficients of all reactants and products, do they have balance number on each atoms? If the number of the atoms in the reactants and products all the same you did the right thing.
2. Convert units of given substance to moles
You should find in the problem what is the given substance that can convert into moles, this substance always have at least 2 variables such as mass with Mr (relative molecular mass), Molarity with volume, or the number of atoms or molecules, or anything variables. The moles of this given substance become the main role on counting the other substance.
3. Find moles of wanted substance using mole ratio in the balance equation
Take the moles of given substance under the same substance in the balanced equation and by using the reaction coefficients compare the coefficient of the wanted substance with given substance that we have found the moles and multiply with the moles of the given substance you will get the moles of wanted substance.
4. Convert the moles of wanted substance into the desired units
After you found the moles of wanted substance you can convert to desired units that the problem wants. If you need to solve it in the mass unit you just multiply the moles with the Mr (relative molecular mass), if you need to changes it into concentration unit you just divide the moles by its volume, and if you need to convert it into the number of atoms or molecules you just multiply by Avogadro’s number.
That is, its very easy isn’t it?
October 26th, 2008
Chemistry Tutor Site-Homework Help
How are the elements in the periodic table arranged?
Chemistry Tutor Site-Answer
Earlier attempts to list the elements to show the relationships between them had usually involved putting them in order of atomic mass. Mendeleev’s key insight in devising the periodic table was to lay out the elements to illustrate recurring (”periodic”) chemical properties (even if this meant some of them were not in mass order), and to leave gaps for “missing” elements. Mendeleev used his table to predict the properties of these “missing elements”, and many of them were indeed discovered and fit the predictions well.
With the development of theories of atomic structure (for instance by Henry Moseley) it became apparent that Mendeleev had listed the elements in order of increasing atomic number (i.e. the number of protons in the atomic nucleus). This sequence is nearly identical to that resulting from ascending atomic mass.
In order to illustrate recurring properties, Mendeleev began new rows in his table so that elements with similar properties fell into the same vertical columns (”groups”).
With the development of modern quantum mechanical theories of electron configuration within atoms, it became apparent that each horizontal row (”period”) in the table corresponded to the filling of a quantum shell of electrons. In Mendeleev’s original table, each period was the same length. Modern tables have progressively longer periods further down the table, and group the elements into s-, p-, d- and f-blocks to reflect our understanding of their electron configuration.
In printed tables, each element is usually listed with its element symbol and atomic number; many versions of the table also list the element’s atomic mass and other information, such as its abbreviated electron configuration, electronegativity and most common valence numbers. As of 2005, the table contains 116 chemical elements whose discoveries have been confirmed. Ninety are found naturally on Earth, and the rest are synthetic elements that have been produced artificially in particle accelerators. Elements 43 (technetium) and 61 (promethium), although of lower atomic number than the naturally occurring element 92, uranium, are synthetic; elements 93 (neptunium) and 94 (plutonium) are listed with the synthetic elements, but have been found in trace amounts on earth.
October 20th, 2008
Chemistry Tutor Site-Homework Help
Why does reactivity decrease as you descend the column of Halogens?Study case why is Fluorine more reactive than Chlorine?
Chemistry Tutor Site-Answer
The exact answer to you question is: the general reactivity of halogens (to form ionic compounds) decreases down the group.
when those elements goes down the group, the number of filled electron shells increases. due to the screening effect, the attraction between the nucleus and the coming electron (donated by the cation, eg, Na donates an electron to Br, forming Na+ and Br-) is reduced down the group.
since electrons are less easy to be accepted by the halogen atoms, the tendency for the atoms to form ions decreases down the group. therefore the reactivity decreases down the group.
As the atomic structure of the halogens becomes more complex with increasing atomic weight there is a gradiation in physical properties. For example: Fluorine is a pale green gas of low density. Chlorine is a greenish-yellow gas 1.892 times as dense as fluorine. Bromine is a deep reddish-brown liquid which is three times as dense as water. Iodine is a grayish-black crystalline solid with a metallic appearance. And astatine is a solid with properties which indicate that it is somewhat metallic in character.
Properties
The halogens show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point. Not all halogens react with the same intensity. Fluorine is actually the most reactive and combines all of the time. As you move down the column, reactivity decreases.
October 20th, 2008
Chemistry Tutor Site-Homework Help
What’s the difference between an atom and an element?
Chemistry Tutor Site-Homework Help
A atom is a concept, which has a structure consisting of nucleus with has protons and neutrons in it, and is surrounded by electrons in discrete energy levels that are described by the size and complexity of the atom.
An element is a PARTICULAR atom. It has a fixed number of protons, electron, and neutrons, as well as isotopes.
So any atomic structure can be an element, but only one element can exist with a particular atomic structure.
An Atom is the smallest part of an Element that keeps all its properties. A Element is a substance composed of atoms having an identical number of protons in each nucleus. Elements cannot be reduced to simpler substances by normal chemical means.
October 20th, 2008
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