Mass Percent
What is the mass percent of carbon, hydrogen, and oxygen in pure ethanol C2H6O?
First, we calculate the mass of one mole of C2H6O...| mass of 2 moles of C is | 2 X 12.011 g |
| mass of 6 moles of H is | 6 X 1.008 g |
| mass of 1 mole of O is | 15.999 g |
|
Molecular weight of C2H6O is |
46.069 g |
Next we calculate the mass percents
Note that the mass percentages should add up to 100%.
Combustion Analysis
When chemists make new compounds, one of the first things they often do is determine the mass % for the different elements in the compound. To analyze the mass percent of carbon and hydrogen, chemists use a combustion device.
The sample is burned in the presence of excess oxygen, which converts all the carbon to carbon dioxide and all the hydrogen to water. The CO2 and H2O produced are absorbed in two different stages, and their masses were determined by measuring the increase in weight of the absorbers.
Ascorbic acid (vitamin C) contains only C, H, and O. Combustion of 1.000 g of Ascorbic acid produced 40.9% C and 4.5% H. What is the empirical formula for Ascorbic Acid?
First, we need to calculate the mass percent of Oxygen. Since the sample contains C, H, and O, then the remaining
Now we need to express the composition in grams and determine the number of moles of each element:
Next, we divide by the smallest number of moles to obtain the mole ratio, which is also the atom ratio. In this case, carbon has the smallest number of moles, so...
| C: | 0.0340 moles/0.0340 moles | = | 1 |
| H: | 0.045 moles/0.0340 moles | = | 1.32 ~ 1 1/3 |
| O: | 0.0341 moles/0.0340 moles | ~ | 1 |
Finally, we calculate the smallest whole integer ratios by multiplying each number above by 3 to get
| C: 3 | H:4 | O:3 |
thus we obtain the empirical formula C3H4O3
Remember the empirical formula has the smallest whole integer ratios; the molecular formula can be different, e.g., C6H8O6, or C9H12O9, or C12H16O12, ... are all possible molecular formulas.
Now let's look at a related question.
What is the molecular formula if the molecular weight of Ascorbic Acid was formed to be 176 g/mole?
In this case, we need to find the multiplication factor between the molecular formula and the empirical formula:
The empirical formula weight of Ascorbic Acid is
| mass of 3 moles of C is | 3 X 12.011 g |
| mass of 4 moles of H is | 4 X 1.008 g |
| mass of 3 moles of O is | 3 X 15.999 g |
|
mass of 1 mole of CH4 is |
88.062g |
Therefore, the multiplicative factor is (176 g/mole)/(88.062 g/mole) ~ 2, and the molecular formula for Ascorbic Acid is C6H8O6
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