Here is a more complicated example:
What is the percent composition of calcium acetate?
To solve this, we must first write the formula for calcium acetate. If you need a refresher on writing formulas, you can go back to nomenclature.
Calcium acetate is Ca(C2H3O2)2
So, the question is really asking what are the percentages of Ca, C, H and O.
To solve this we set up the fractions below:
If you aren’t sure why we used 4 times the molar mass of C (and 6 of H and 4 of O) take a look at the formula. Each molecule of calcium acetate contains 2 acetate ions and each acetate ion contains 2 carbons. So 2x2=4 C atoms per molecule of calcium acetate.
Substituting in the values for the variables above, gives:
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