Question

Give an example of a case in which two molecules have different molecular formulas but the same empirical formula.

Short answer

Ethene \(C_2H_4\) and butene \(C_4H_8\) have different molecular formulas but the same empirical formula \(CH_2\).

Step-by-step solution

Understanding Molecular Formulas

Molecular formulas represent the actual number of atoms of each element in a molecule. For example, glucose has the molecular formula \(C_6H_{12}O_6\).

Understanding Empirical Formulas

Empirical formulas indicate the simplest whole-number ratio of atoms in a molecule. For glucose \(C_6H_{12}O_6\), the empirical formula is \(CH_2O\) by dividing each subscript by their greatest common divisor, 6.

Providing Example Molecules

Consider the molecules ethene \(C_2H_4\) and butene \(C_4H_8\). Ethene's molecular formula is \(C_2H_4\) and butene's is \(C_4H_8\).

Calculating Empirical Formula for Ethene

The empirical formula for ethene \(C_2H_4\) is \(CH_2\). The ratio of carbon to hydrogen is 1:2.

Calculating Empirical Formula for Butene

The empirical formula for butene \(C_4H_8\) is \(CH_2\) as well. The ratio of carbon to hydrogen is also 1:2.

Confirming the Example

Both ethene and butene have the same empirical formula, \(CH_2\), even though their molecular formulas are different.

Key concepts

Molecular Formula

The molecular formula of a compound gives us a detailed look at the composition of a molecule. It tells the exact number of atoms of each element present. For example, the molecular formula for glucose is \(C_6H_{12}O_6\). This means glucose is made up of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

In essence, the molecular formula reveals the true, detailed structure of a molecule. This makes it crucial when attempting to understand and decipher the complete makeup of complex molecules.

Ratio of Atoms

The ratio of atoms in a chemical formula is all about comparing the numbers of each type of atom present in a molecule. When we talk about this ratio, we are often referring to the empirical formula, where the focus is on the simplest whole number ratio.

For instance, take the empirical formula \(CH_2\). This means for every one carbon atom, there are two hydrogen atoms. Regardless of the complexity of the molecule, this basic ratio remains a core characteristic. This is why the same empirical formula can apply to completely different molecules, such as ethene \(C_2H_4\) and butene \(C_4H_8\). Both have an empirical formula with a carbon to hydrogen ratio of 1:2.

Chemical Nomenclature

Chemical nomenclature is the standardized system for naming chemical compounds. It's like a language for chemists, ensuring we all understand each other when discussing molecules. Molecular and empirical formulas are integral parts of this system, providing clear and precise ways to describe what a compound is made of.

Pioneers in chemistry created these naming conventions to avoid confusion and mistakes. For instance, knowing the difference between ethene \(C_2H_4\) and butene \(C_4H_8\) comes down to having a firm grasp of chemical nomenclature. When these names are used, one can easily deduce molecular structures from their molecular formulas.

Empirical Formula Calculation

Empirical formula calculation involves simplifying the molecular formula to its most basic ratio. To achieve this, divide the subscripts in the molecular formula by their greatest common divisor.

Let's look at glucose where the molecular formula is \(C_6H_{12}O_6\). By dividing each subscript—6, 12, and 6—by 6, we obtain the empirical formula \(CH_2O\). This calculation captures the simple ratio of elements within the compound.

Understanding this process helps identify when different molecules might share the same empirical formula. The molecules ethene \(C_2H_4\) and butene \(C_4H_8\) both simplify to \(CH_2\), displaying the shared fundamental ratio despite differing molecular compositions.