Mass to Mass Conversions

  In order to complete mass to mass conversions, there is one additional conversion you have to make that involves moles.

EX.1: How many grams of O2 are there when 10 g of Al2O3 decomposes?

1) To start off, create a balanced equation.

Al2O3  ->  Al + O2

2Al2O3  ->  4Al + 3O2

2) Since you are given the mass of Al2O3, use the molar mass of the compound in order to cancel out the grams and left with moles

10 g of Al2O3   x   1 mol Al2O3

                           102.0 g Al2O3

3) Next, you will have to cancel out the moles of Al2O3. One way to look at the co-efficients of a compound is to think of them as moles, which you can use to cancel out Al2O3. Put what you know on the bottom and put what you need on the top.

10 g of Al2O3   x   1 mol Al2O3   x   3 mol O2

                           102.0 g Al2O3      2 mol Al2O3

4) Now that you have the moles of O2, you can now use molar mass as a conversion factor in order to find out what you're looking for which the mass of O2.

10 g of Al2O3   x   1 mol Al2O3   x   3 mol O2    X   32.0 g O2

                           102.0 g Al2O3      2 mol Al2O3     1 mol O2

5) Finally, calculate your answer and leave it in its significant digits.

10 g of Al2O3   x   1 mol Al2O3   x   3 mol O2    X   32.0 g O2   =   4.7 g of O2

                           102.0 g Al2O3      2 mol Al2O3     1 mol O2

For more examples about mass to mass or moles to mass conversions, check out this video:

http://www.youtube.com/watch?v=sTsshxcHhaI