Elemental composition of pure substances
Determine empirical and molecular formulas by measuring mass percentages of constituent elements. Perform composition analysis of pure compounds via percent by mass.
▶ Empirical, molecular, and structural formulas
There are three main types of chemical formulas: empirical, molecular and structural. Empirical formulas show the simplest whole-number ratio of atoms in a compound, molecular formulas show the number of each type of atom in a molecule, and structural formulas show how the atoms in a molecule are bonded to each other.
▶ Worked example: Calculating mass percent
To calculate the mass percent of an element in a compound, we divide the mass of the element in 1 mole of the compound by the compound's molar mass and multiply the result by 100.
▶ Worked example: Determining an empirical formula from percent composition data
We can use percent composition data to determine a compound's empirical formula, which is the simplest whole-number ratio of elements in the compound.
▶ Worked example: Determining an empirical formula from combustion data
In combustion analysis, an organic compound containing some combination of the elements C, H, N, and S is combusted, and the masses of the combustion products are recorded. From this information, we can calculate the empirical formula of the original compound.
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