Balancing Equations and Reactions with Compound Units
When working with chemical notation, it has been established that the subscripts (or numbers FOLLOWING the elements) represent the number of those elements present. The coefficient tells how many of the molecule is present.
3 H20 or 3 H2O
means 3 molecules of water in which are 2 atoms of hydrogen and 1 atom of oxygen.
There is another variation of this notation that is applied in certain cases with some compounds. Because of how the compounds form, there is sometimes a value in keeping some of the elements as a unit and subscripting the whole unit to show how many of that unit are present.
Look at the reaction below:
CaC2 + 2 H2O ---> Ca(OH)2 + C2H2
Notice on the product side, the OH is inside parenthesis. This represents that that is a unit of molecules that are being kept together based on how the compound is formed. The subscript indicates that there are two of these units present.
3 Ca(OH)2 has in it, 3 Ca, 6 O, and 6 H. (The subscripted 2 applies to both atoms inside the parenthesis, and the coefficient of 3 applies to the whole molecule.)
Ca3(PO4)2 has in it, 3 Ca, 2 P, and 8 O. (The subscripted 2 applies to the PO4, so there are 2 P and 8 0)
2 Cu(NO3)2 has—to begin with, there are 2 molecules of Cu(NO3)2 as indicated by the coefficient.
EACH molecule has 1 Cu and 2 (NO3). Since there are 2 (NO3) (The subscript 2 applies to everything inside the parenthesis.), that means there are 2 N and 6 O in each Cu(NO3)2 molecule.
Since there are 2 molecules of Cu(NO3)2, there are in TOTAL:
Coefficients, the numbers in front, apply to the whole molecule and tell how many molecules or "sets" of molecules are present.
Subscripts (or number FOLLOWING the atom symbols) tell how many of that atom are in the molecule.
If a group of atoms are inside parenthesis:
They are to be kept together as a unit.
Any subscripts to the closing parenthesis means that there are that many units of the atoms inside the parenthesis are present.