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Chemical Formula

The Chemical Formula is an indication of the makeup of a compound in terms of the kinds of atoms and their relative numbers. From the knowledge of oxidation numbers and valence and an understanding of atomic structure, it is possible to write chemical formulas

A chemical formula is a "notation made up of the chemical symbols of the elements present in a compound and numerical subscript that indicate the number of atoms of each element present in a molecule of the compound."A chemical formula provides information about the atomic composition of a substance.

Chemical Formula - Written combination of element symbols that represents the atoms combined in a chemical compound.

In order to write formulas correctly one must follow the capitalization rules for elemental symbols. Making error of capitalizing the second letter of an elements symbol can dramatically alter the meaning of a chemical formula. When we attempt to write a formula it is important to know whether the substance actually exists.

Before writing the chemical formula we need the symbols of elements. The symbols of elements are taken from periodic table.

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Chemical Problems

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Solved problems based on molar mass and molecular mass are given below.

Solved Examples

Question 1: For each of the following chemical formulas, determine how many atoms of each element are present in one molecule of the compound.
  1. HCN - hydrogen cyanide a poisonous gas
  2. C18H21NO3 - codeine a pain killing drug
  3. Ca10(PO4)6(OH)2 - hydroxyapatite, present in tooth enamel

  1. One atom each of the elements hydrogen, carbon and nitrogen is present. Remember that the subscript 1 is implied when no subscript is written.
  2. This formula indicates that 18 carbon atoms, 21 hydrogen atoms, 1 nitrogen atom and 3 oxygen atoms are present in one molecule of the compound.
  3. There are 10 calcium atoms. The amounts of phosphorous, hydrogen and oxygen are affected by the subscripts outside the parentheses. There are 6 phosphorous atoms and 2 hydrogen atoms present. Oxygen atoms are present in two locations in the formula. There are total 26 oxygen atoms: 24 from the PO4 subunits(6x4) and 2 from the OH subunit (2x1).


Question 2: Write the correct chemical formulas for the compounds containing sulfur whose molecular pictures are as follows.

Chemical formulas follow specified guidelines. We must use the correct symbols and subscript and place the symbols in the correct order.
  • Oxygen and sulfur are in same group of the periodic table. According to the third guideline, the lower element appears first. Thus the correct formula is SO3.
  • This compound contains hydrogen and sulfur an element from Group 16 of the periodic table. According to the second guideline hydrogen appears before elements from this group. Thus the correct formula is H2S.
  • The elements in this compound fall in two different groups of the periodic table. According to the first guideline, the element farther to the left appears first. Thus the correct formula is SF4.
  • Only one element is present in this substance, whose molecules contain eight atoms. Thus the correct formula is S8.


More topics in Chemical Formula
Percent Composition Formula Mole Fraction Formula
Degree of Unsaturation Formula pH Formula
Ionic Compound Formula Solubility Formula
Conservation of Energy Formula
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