Chemistry / Unit 3 Chem 0385 worked example, 60 to 90 seconds
Calculating Percent Composition from Experimental Mass Data
The idea
Calculating percent composition from experimental data means dividing each element's measured mass by the sample's total mass, then converting each result to a percentage.
In practice
A sample known to contain only carbon, hydrogen, and nitrogen is analyzed and found to hold 7.34 g carbon, 1.85 g hydrogen, and 2.85 g nitrogen, for a total sample mass of 12.04 g. Dividing each element's mass by 12.04 g and multiplying by 100 gives about 60.9% carbon, 15.4% hydrogen, and 23.7% nitrogen. Checking that the three percentages sum to almost exactly 100% confirms no element's mass was left out of the total.
Where people slip
Forgetting to add every element's mass into the total before dividing produces percentages that do not sum to 100%, a mistake the summing check catches immediately.
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