Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019
Percent Composition of Compounds
· To learn to find the mass percent of an element in a given compound.
So far we have discussed the composition of compounds in terms of the numbers of constituent atoms. It is often useful to know a compound’s composition in terms of the masses of its elements. We can obtain this information from the formula of the compound by comparing the mass of each element present in mole of the compound to the total mass of mole of the compound. The mass fraction for each element is calculated as follows:
The mass fraction is converted to mass percent by multiplying by .
We will illustrate this concept using the compound ethanol, an alcohol obtained by fermenting the sugar in grapes, corn, and other fruits and grains. Ethanol is often added to gasoline as an octane enhancer to form a fuel called gasohol. The added ethanol has the effect of increasing the octane of the gasoline and also lowering the carbon monoxide in automobile exhaust.
Note from its formula that each molecule of ethanol contains two carbon atoms, six hydrogen atoms, and one oxygen atom. This means that each mole of ethanol contains moles of carbon atoms, moles of hydrogen atoms, and mole of oxygen atoms. We calculate the mass of each element present and the molar mass for ethanol as follows:
The mass percent (sometimes called the weight percent) of carbon in ethanol can be computed by comparing the mass of carbon in mole of ethanol with the total mass of mole of ethanol and multiplying the result by .
That is, ethanol contains by mass of carbon. The mass percents of hydrogen and oxygen in ethanol are obtained in a similar manner.
The mass percents of all the elements in a compound add up to , although rounding-off effects may produce a small deviation. Adding up the percentages is a good way to check the calculations. In this case, the sum of the mass percents is .
Interactive Example 8.9. Calculating Mass Percent
Carvone is a substance that occurs in two forms, both of which have the same molecular formula and molar mass. One type of carvone gives caraway seeds their characteristic smell; the other is responsible for the smell of spearmint oil. Compute the mass percent of each element in carvone.
Where Are We Going?
We want to determine the mass percent of each element in carvone.
What Do We Know?
· The formula for carvone is .
· We know the atomic masses of carbon ( g/mol), hydrogen ( g/mol), and oxygen ( g/mol).
· The mass of isopentyl acetate is g.
· There are molecules in mole.
What Do We Need to Know?
· The mass of each element (we’ll use mole of carvone)
· Molar mass of carvone
How Do We Get There?
Because the formula for carvone is , the masses of the various elements in mole of carvone are
Next we find the fraction of the total mass contributed by each element and convert it to a percentage.
Reality Check Add the individual mass percent values—they should total within a small range due to rounding off. In this case, the percentages add up to .
Self-Check: Exercise 8.7
· Penicillin, an important antibiotic (antibacterial agent), was discovered accidentally by the Scottish bacteriologist Alexander Fleming in 1928, although he was never able to isolate it as a pure compound. This and similar antibiotics have saved millions of lives that would otherwise have been lost to infections. Penicillin, like many of the molecules produced by living systems, is a large molecule containing many atoms. One type of penicillin, penicillin F, has the formula . Compute the mass percent of each element in this compound.
See Problems 8.45, 8.46, 8.47, 8.48, 8.49, and 8.50.