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# Why can't I seem to understand how to balance chemical equations?

I don't understand at all how to balance chemical equations. I need to pass my CHM100 class this summer to get my dental hygiene diploma. I am taking my CHM class online and am not doing well. What I don't understand about balancing chemical equations is where the coefficients come from? Our test is going to be formatted like so:

___Al +___O2-----> ___ Al2O3 and I do not get how it is balanced as

4 Al + 3 O2 -----> 2 Al2O3

Where does the 4, 3, and 2 come from???

We're also going to have to write and balance chemical equations for reactions, like:

Iron metal and sulfur powder are heated in the flask producing solid iron (II) sulfide

which is Fe + S ---> FeS

I don't understand why there are no subscripts or coefficients?

Any help would be much appreciated!

### 2 Answers

- geek.....Lv 71 decade agoFavorite Answer
rules to balancing eqn:

step 1: balance least element first

step 2: balance O and H last

step 3: use coefficient if necessary

step 4: reactant = product

good luck.

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- Anonymous1 decade ago
First Problem:

1)Always check the charge on the element (or compound).

In the first problem: 1 Al has a 3+ charge and 1 O has a 2- charge.

Therefore Al2 = 6+ charge and O3 = 6- charge, making Al2O3 = zero charge

2)The number of reactants must equal the number of products (be balanced).

The 4Al reactant is equal to the 2Al2 product. (4 aluminums on each side)

The 3O2 reactant is equal to the 2O3 product. (6 oxygens on each side)

As for the second problem:

1)Again, always check the charge of the element.

Looking at FeS: S has a charge of 2- (you know this by looking at the periodic

table of elements; S has 6 electrons and will therefore GAIN 2 more, giving it a

total charge of 2-). Since S is 2- then Fe must be 2+ because the total charge of the

compound must equal zero.

2)Therefore FeS ---->Fe + S. The equation is balanced and doesn’t need

subscripts/coefficients. There is 1 Fe on each side and 1 S on each side.

I hope this helps a little!

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