What's the difference between empirical formula and molecular formula?
Molecular formulas tell you how many atoms of each element are in a compound. It is the full formula of a molecule.
Empirical formulas tell you the simplest or most reduced ratio of elements in a compound.
What is empirical formula and why use it?
- Empirical formula is simplified to its lowest ratio and doesn't show you the actual number of atoms in the molecule.
- In other words, it only show your the relative amount or proportion of each element in the compound.
- To find the empirical formula, simply divide the subscripts by the smallest number.
- There are many uses for empirical formula, one example is checking the purity of a manufactured compound such as vitamins making sure the elements are in the correct proportion. If the result isn't what's expected, then the product is either contaminated or something has gone wrong with the production. Another use is in forensic analysis. Blood sample, fibers, bullets, and finger prints are analysed. In forensic science, chemical analysis is performed using mole conversion, percent composition and empirical formula to determine DNA structure and the samples from actual crime scenes are compared with suspects' samples.
- You cannot use empirical formula to determine an unknown compound because it doesn't give you details of the molecular structure. But if you have the molecular mass of the compound, you can eventually derive it's molecular formula.
Formulas of ionic compounds are always the same as their empirical formulas?
It might surprise you that ionic compounds are strictly not considered molecules because they are not chemically bonded like molecules.
- Ionic compounds consist of independent charged atoms, called ions.
- The oppositely charged ions are attracted to each other like magnets.
- They form a net zero charge; the positive charges of cations cancel out the charges of the anions.
- For example, sodium chloride is made of Na(+) and Cl(-) ions. A compound of NaCl has neutral charge.
- When sodium chloride dissolves, it completely dissociates into separate ions.
Why sugar is a proper molecule.
- It is chemically bonded together so when it is dissolved in water, the molecules don't break apart.
- Each C12H22O11 will remain as a whole molecule in water, though the molecules themselves might separate from each other.
Ionic compounds do not exist as molecules and, as a result, do not have molecular formulas.
Molecular formula can sometimes be equal to the empirical formula
Sometimes, a molecular formula is already reduced to it's simplest form. In this case, it is the same as its empirical formula.
Examples of such molecules are carbon monixide (CO), carbon dioxide (CO2), and nitrogen dioxide (NO2).
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