Calculations with Chemical Equations

Now that we understand the mole concept and mol-mol conversion factors, we're ready to do quantitative calculations. Consider the rusting of oxygen:

Suppose we have a brand new iron nail and find that it has a mass of 43.6 g. We let it rust away in air. If the entire nail rusts away, what will be the mass of the rust (Fe2O3)? We can solve this problem in one of two ways:

Method I:
 Step 1

Determine moles of iron:
 Step 2

 Convert from moles of iron to moles of rust:
 Step 3

 Convert from moles of rust to mass of rust:

Method 2: Steps 1-3 above can be accomplished in one overall calculation by treating the problem like a dimensional analysis problem:

The second method is somewhat easier since it can be accomplished in one line. Let us do one more example:

Consider the combustion of butane:

If we need to recover 150 g of water, what mass of butane is necessary? The method 2 result is:

Most chemical calculations involving masses are carried out in a similar manner.

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C101 Class Notes
Prof. N. De Leon