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A chemical equation is a description of a chemical reaction.
For example, this chemical equation describes the chemical reaction of
changing sodium hydroxide and hydrogen chloride into salt (sodium chloride) and water:
NaOH + HCl ==> NaCl + H2O
The equation above was a fairly simple equation, both sides were already balanced.
Balanced means that all the atoms present among the reactants (the left side of the equation) were accounted for among the products (the right side).
Now lets take a simple unbalanced equation and try to balance it.
This is the reaction of turning oxygen and hydrogen into water (unbalanced):
H2 + O2 ==> H2O
Notice that there are two hydrogens and two oxygens on the left side.
But there are only two hydrogens and one oxygen on the right side.
For the equation to be correct, it must be balanced, so both sides have equal amounts of atoms.
To do that, we must adjust the coefficients of the equation.
When the equation is finally balanced, it is:
2H2 + O2 ==> 2H2O
Now on both sides, there are four hydrogen atoms and two oxygen atoms.
The equation now states, for every two moles of hydrogen gas, two moles of water can be formed,
and for every one mole of oxygen gas, two moles of water can be formed.
Mass to Mass problems
In balanced equations the mole ratio of products to reactants always allows you to calculate every substance in the equation if you know that every other substance in the reaction is completely consumed.
To figure out mass to mass problems, change each formula mass into the equivalent mole ratios.
Mass to Volume problems
When one given is volume and the other is mass the problem can be solved by converting both to mole units then to the desired state (i.e. solid, liquid, gas).
Volume to Volume problems
Avogadro's law states that all gases at STP occupy the same volume, therefore it is only required to divide the volume of every gas by 22.4 liters to obtain the correct number of moles.