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From Wikipedia

Chemical formula

A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound.

The chemical formula identifies each constituent element by its chemical symbol and indicates the number of atoms of each element found in each discrete molecule of that compound. If a molecule contains more than one atom of a particular element, this quantity is indicated using a subscript after the chemical symbol (although 18th-century books often used superscripts) and also can be combined by more chemical elements. For example, methane, a small molecule consisting of one carbon atom and four hydrogen atoms, has the chemical formula CH4. The sugar molecule glucose has six carbon atoms, twelve hydrogen atoms and six oxygen atoms, so its chemical formula is C6H12O6.

Chemical formulas may be used in chemical equations to describe chemical reactions. For ionic compounds and other non-molecular substances an empirical formula may be used, in which the subscripts indicate the ratio of the elements.

The 19th-century Swedish chemist Jöns Jakob Berzelius worked out this system for writing chemical formulas.

Molecular geometry and structural formulas

The connectivity of a molecule often has a strong influence on its physical and chemical properties and behavior. Two molecules composed of the same numbers of the same types of atoms (i.e. a pair of isomers) might have completely different chemical and/or physical properties if the atoms are connected differently or in different positions. In such cases, a structural formula can be useful, as it illustrates which atoms are bonded to which other ones. From the connectivity, it is often possible to deduce the approximate shape of the molecule.

A chemical formula supplies information about the types and spatial arrangement of bonds in the chemical, though it does not necessarily specify the exact isomer. For example ethane consists of two carbon atoms single-bonded to each other, with each carbon atom having three hydrogen atoms bonded to it. Its chemical formula can be rendered as CH3CH3. In ethylene there is a double bond between the carbon atoms (and thus each carbon only has two hydrogens), therefore the chemical formula may be written: CH2CH2, and the fact that there is a double bond between the carbons is implicit because carbon has a valence of four. However, a more explicit method is to write H2C=CH2 or less commonly H2C::CH2. The two lines (or two pairs of dots) indicate that a double bond connects the atoms on either side of them.

A triple bond may be expressed with three lines or pairs of dots, and if there may be ambiguity, a single line or pair of dots may be used to indicate a single bond.

Molecules with multiple functional groups that are the same may be expressed by enclosing the repeated group in round brackets. For example isobutane may be written (CH3)3CH. This semi-structural formula implies a different connectivity from other molecules that can be formed using the same atoms in the same proportions (isomers). The formula (CH3)3CH implies a central carbon atom attached to one hydrogen atom and three CH3 groups. The same number of atoms of each element (10 hydrogens and 4 carbons, or C4H10) may be used to make a straight chain molecule, butane: CH3CH2CH2CH3.

The alkene but-2-ene has two isomers which the chemical formula CH3CH=CHCH3 does not identify. The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond (cis or Z) or on the opposite sides from each other (trans or E).

Polymers

For polymers, parentheses are placed around the repeating unit. For example, a hydrocarbon molecule that is described as CH3(CH2)50CH3, is a molecule with fifty repeating units. If the number of repeating units is unknown or variable, the letter n may be used to indicate this formula: CH3(CH2)nCH3.

Ions

For ions, the charge on a particular atom may be denoted with a right-hand superscript. For example Na+, or Cu2+. The total charge on a charged molecule or a polyatomic ion may also be shown in this way. For example: hydronium, H3O+ or sulfate, SO42−.

For more complex ions, brackets [ ] are often used to enclose the ionic formula, as in [B12H12]2−, which is found in compounds such as Cs2[B12H12]. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in [Co(NH3)6]3+. Here (NH3)6 indicates that the ion contains six NH<


From Yahoo Answers

Question:An Oxide of gold thermally decomposes. A 0.204-g sample of the oxide is heated, forming 0.182 g of gold. What is the Empirical formula of the gold oxide? Thank you.

Answers:find moles using molar mass: 0.182 g gold @ 197 g/mol= 0.00092 moles Au 0.022 g oxygen @ 16 g/mol = 0.001375 moles of Oxygen divide by the smallest moles to get the mole ratio: 0.00092 moles Au / 0.00092 = 1 mole Gold 0.001375 moles of Oxygen / 0.00092 = 1.5 mol Oxygen which doubles to give a whole number ratio: your answer is Au2O3

Question:for chemistry need help

Answers:alkanes - 2n+2 alkenes - 2n alkynes - 2n-2 where n is the number of carbons and the above calculation yields the number of hydrogens

Question:Just to get an idea of what Chem 1 is like COMPARED TO introductory chemistry, and ways to prepare for chem 1 ?

Answers:About the hardest problems you will have to do is form chemical formula compouds basic adding and subtracting but a lot of it.

Question:I do not understand why something is acidic and why something is basic. We all know that R-COOH can donate its hydrogen when under basic conditions but why can't R-NH2 donate a hydrogen under the same basic conditions Vice versa, why can't R-COOH readily accept a hydrogen in acidic conditions like R-NH2 even though the oxygens have lone pairs to accept a hydrogen just like how amines have lone pairs as well? Do we just have to know the pKa charts? Is there any other explanation than Ka?

Answers:Alrighty the answer lies within the stability of the atom that will be accepting or donating a hydrogen. Note: the terms hydrogen, hydrogen ion and proton are used interchangably, although the last two are more technically correct. An oxygen can't have more than two bonds, because of its valency of -2. R-COOH has the formula of: O II C -- OH As you can see, each O has two bonds, and cannot accept another hydrogen (there are some circumstances in more advanced organic chemistry in which it can, but don't concern yourself with this). However, O is a really electronegative atom, and is happy having only one bond and a negative charge, making the following possible: O II C -- O- <--- this O has a negative charge As you can see, RCOOH can donate a proton but can accept one. Note: Carbon cannot accept the proton because it can only have four bonds (see valency). Similarly, the nitrogen in RNH2 can have four bonds, but not two. If the N accepts a proton, it will become RNH3+ (the ammonium cation). Hope this clears it up for you!

From Youtube

Chemistry Tutorial 13.1b: Hydrocarbons Molecular Formulas :This video shows how to get the formulas for alkanes,m alkenes and alkynes using the general formulas.