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Understand and Interpret Chemical Formulas

Before we can proceed to balance chemical equations, we must first know how to interpret a chemical formula in terms of the number of atoms of each element.


How many atoms of each element are in the following chemical formulas?

a) CO2
b) 2NH3
c) 2CH3OH
d) Ba(NO3)2
e) 2Al2(SO4)3


a) CO2

  • The C indicates that it has one carbon atom.
  • The O2 indicates that it has two oxygen atoms.
  • Note that the subscript, in this case 2, is only for the oxygen atom just before it. It does not affect the carbon atom.

b) 2NH3

  • For each NH3 molecule, the N indicates that it has one nitrogen atom and the H3 indicates that it has three hydrogen atoms.
  • The number 2 in front (called the coefficient) indicates that there are two NH3 molecules.
  • So, in total, there are two nitrogen atoms and six hydrogen atoms.

c) 3CH3OH

  • Each CH3OH molecule has one carbon atom, four hydrogen atoms and one oxygen atom.
  • The coefficient of 3 in front indicates that there are three CH3OH molecules.
  • So, in total, there are three carbon atoms, twelve hydrogen atoms and three oxygen atoms.

d) Ba(NO3)2

  • If you have a bracket then the subscript must be multiplied into all the atoms in the bracket. (NO3)2 would mean that there are two nitrogen atoms and six oxygen atoms.
  • So, in total, there are one barium atom, two nitrogen atoms and six oxygen atoms.

e) 2Al2(SO4)3

  • (SO4)3 would mean that there are three sulfur atoms and twelve oxygen atoms.
  • The coefficient of 2 in front indicates that there are two Al2(SO4)3 molecules.
  • So, in total, there are four aluminum atoms, six sulfur atoms and 24 oxygen atoms.

Atoms can neither be destroyed nor created. Therefore, in a chemical reaction

The sum of atoms before reaction = the sum of atoms after reaction

Balancing chemical equations may require some trial and error. There are some general rules that could be helpful, but they may not work all the time.

How to interpret the elemental symbols, subscripts, and parentheses in a chemical formula?
A chemical formula is an expression of the types and number of atoms in a substance.
Some chemical formulas use parentheses to clarify atomic composition.
Determine the number and types of atoms in the following chemical formulas:
1. Mg3(PO4)2
2. C2H4(OH)2
3. AL2(S2O3)3
Understanding Chemical Formulas
Understand how subscripts and coefficients work when using chemical formulas. How to determine how many of each atom there are in a given chemical compound/molecule?
A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes and brackets.
Determine how many of each atom there are in the following compound.

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