Balancing Chemical Equations Problem- Al + O2 -------> Al2O3 ( Please put steps to complete in "9th grade terms") :)

Question

I don't understand!!

Answer 1

You need to think of a certain number of Al atoms, and a (possibly different) number of O2 molecules, such that when they are combined, the result is some multiple of Al2O3 molecules.

 

I find it easiest to work this backwards.  First, consider how many Al's and O's are in increasing amounts of Al2O3.  For example:

 

1*Al2O3 = Al2O3

2*Al2O3 = Al4O6

3*Al2O3 = Al6O9

...etc

 

What we are looking for is quantities of Al and O in our final product of Al2O3 that can be gotten from the input ingredients.  Consider the final product 1*Al2O3 = Al2O3.  This product has three O atoms, but we are only allowed to add an even number of O atoms as input.  Therefore, there is no way that we can use Al and O2 to create a single molecule of Al2O3.

 

Consider the final product 2*Al2O3 = Al4O6.  We can get six oxygen atoms by inputting 3 of the O2 molecules.  And we are not constrained in how we add Al, since we can add this atom 1-at-a-time.

 

Hence, 4*Al + 3*O2 = 2*Al2O3.

 

Notice that the problem is very similar to that of finding a Least Common Multiple.  The Least Common Multiple of 2 (from the O2 input) and 3 (from the O3 product) is 6, and this is how many O atoms must be in the final product.

 

The best approach in these problems is to identify the contributing atom that has the largest Least Common Multiple between its input ingredient and output product.  If there are multiple restrictions (due to more atom types), you may need to find the Least Common Multiple among many terms.  I hope this helps.  Good luck!

 

Paul