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Are you struggling to understand your new chemistry course? Begin with the empirical formula – it's an excellent starting point. Image by Klaus Vedfelt / Getty ImagesIn the field of chemistry, scientists use various techniques to identify chemical compounds, such as molecular formulas, molar masses, and molecular diagrams. The simplest of these is the empirical formula.
When consulting a textbook or reference guide, empirical formulas give us insight into a compound's chemical structure, but they don't always provide the exact number of atoms in each molecule.
Clarification on the Microscopic Level
The empirical formula, discovered by scientists in the mid-1800s, marked a breakthrough in chemistry, providing chemists with the first method to describe the behavior of both common and rare materials at the microscopic level.
In the laboratory, chemists would derive empirical formulas for various chemical compounds by isolating the individual elements found on the periodic table and determining the quantities of each element.
Example of a Chemical Compound
For example, sodium chloride (table salt) can be broken down into its elemental components: sodium (Na) and chlorine (Cl). Individually, these elements are unstable and react with the oxygen and water in the surrounding atmosphere.
Though toxic when consumed separately, these elements combine to form a molecular compound that is a vital part of our daily nutrition.
Empirical Formula and Molar Mass
To determine the empirical formula of a compound like table salt, chemists must first trigger a reaction to separate it into its basic atomic components.
Assuming that the entire sample reacts, chemists can weigh the remaining sodium and chlorine to calculate the molar mass, which helps in determining the empirical composition of the compound.
For table salt, the mass ratio of sodium to chlorine is 1:1, resulting in an empirical formula of NaCl, which coincidentally is also the correct chemical formula for the compound.
In the 19th century, the techniques used to break down compounds into their atomic elements and derive the empirical formula were revolutionary, greatly enhancing our understanding of how chemicals are formed and why they display certain properties.
While the empirical formula provides insight, it doesn't fully reveal what's happening inside a molecule. More sophisticated chemical formulas were created to provide a deeper understanding.
Empirical Formula vs. Molecular Formula
In modern laboratories and chemistry studies, it's much more common to encounter the molecular formula of a compound instead of the empirical formula. This is because we've learned that individual molecules tend to be more complex than the simplest whole-number ratio of atoms.
Relying solely on the empirical formula could lead to different compounds sharing the same formula, which makes it less effective for categorizing chemical properties. The total mass of a molecule could be significantly larger than what the empirical formula would imply.
Table salt is an ideal compound for demonstrating empirical formulas, as it contains exactly one sodium and one chlorine atom per molecule. However, not all compounds are so neatly divided, and knowing just the simplest mass ratio inside the molecule doesn't always tell us the exact makeup of a single molecule.
What Molecular Formulas Reveal About a Compound
Consider two chemical compounds, acetylene and benzene. Despite sharing the same empirical formula (CH), meaning one hydrogen atom per one carbon atom, these compounds have different chemical formulas. Acetylene has the formula C2H2, while benzene's formula is C6H6. Compounds with the same empirical formula occur when the chemical formula of one is a multiple of the other.
Though acetylene and benzene have the same basic atomic structure, one benzene molecule contains three times the number of atoms and three times the molar mass compared to an acetylene molecule. While both are hydrocarbons, they serve different purposes.
- Acetylene is used as a high-temperature fuel, especially for tasks like welding.
- On the other hand, benzene is too toxic to be used as a fuel and is instead utilized in the production of industrial plastics.
If chemists or industries were to mistakenly assume that acetylene and benzene were the same compound based only on their empirical formulas, it could result in defects in the products they create, or worse, dangerous situations—such as a welder using a toxic substance as fuel. Properly classifying chemicals is essential to ensure safety and effectiveness.
Determining Molecular Formula Using Empirical Formula
Chemists were initially able to determine a compound's molecular formula by starting with the empirical formula. The number of atoms in a molecular formula is always a whole number, which is found by multiplying the number ratio from the empirical formula. The smallest multiplier for this factor is one.
To calculate the multiplication factor for a specific compound, scientists measured the mole value, or the average mass of an individual molecule. In the case of acetylene and benzene, laboratories were eventually able to calculate the mass of these individual molecules and derive the molecular formulas from that data. Chemicals with entirely different atomic structures can also share the same multiplication factor.
Since we already know both the molecular and empirical formulas for these compounds, we can reverse the process to find the whole number by which the original formula was multiplied. The results appear as follows:
