Question

(a) What is a hydrocarbon? (b) Pentane is the alkane with a chain of five carbon atoms. Write a structural formula for this compound and determine its molecular and empirical formulas.

Short answer

(a) A hydrocarbon is a chemical compound consisting only of hydrogen and carbon atoms, often serving as fuel sources in organic materials like petroleum, natural gas, and coal. (b) The structural formula for pentane is CH3-CH2-CH2-CH2-CH3. Its molecular formula is C5H12, and because it is already in its simplest ratio, the empirical formula is also C5H12.

Step-by-step solution

(a) Defining a hydrocarbon

A hydrocarbon is a chemical compound that consists only of hydrogen and carbon atoms. These compounds can be found in organic materials such as petroleum, natural gas, and coal, and they often serve as fuel sources.

(b) Writing a structural formula for pentane

Pentane, as mentioned in the exercise, is an alkane with a chain of five carbon atoms. To draw its structural formula, we need to connect the carbon atoms in a chain, with each carbon atom bonding to the necessary number of hydrogen atoms. In pentane, each of the two end carbon atoms is bonded to three hydrogen atoms, while the remaining three carbon atoms are each bonded to two hydrogen atoms. So, the structural formula for pentane is as follows: CH3-CH2-CH2-CH2-CH3

Determining the molecular formula for pentane

The molecular formula of a compound shows the actual number of atoms of each element present in the compound. In the case of pentane, there are five carbon atoms and twelve hydrogen atoms. Therefore, the molecular formula of pentane is C5H12.

Determining the empirical formula for pentane

The empirical formula of a compound represents the simplest whole-number ratio of the elements in the compound. Since the molecular formula of pentane is already in its simplest ratio, the empirical formula is the same as the molecular formula: C5H12.

Key concepts

Alkanes

Alkanes are a type of hydrocarbon, which means they are compounds entirely composed of hydrogen and carbon atoms. They fall under the category of saturated hydrocarbons. This signifies that they consist only of single bonds between carbon atoms. The name "alkane" is derived from a Greek word and these molecules have a general formula of $C_n{H}_{2n+2}$, where $n$ represents the number of carbon atoms.

Alkanes are found in many everyday materials, such as gasoline and natural gas. These hydrocarbons are typically not very reactive, but they can burn in oxygen, making them a good fuel source. Each alkane has a specific structure that corresponds to its number of carbon atoms. As alkanes get larger, they change from gases to liquids to solids at room temperature. Pentane is an example of an alkane that exists as a liquid under normal conditions.

Structural Formula

A structural formula provides a visual representation of a molecule, showing how the atoms are arranged and bonded. It reveals the connections between carbon atoms in a hydrocarbon chain and illustrates where other atoms, like hydrogen, are connected. This is crucial for understanding the detailed makeup of the molecule, beyond the simplicity of its molecular formula.

For pentane, the structural formula is written as CH3-CH2-CH2-CH2-CH3, where each dash represents a single bond between atoms. This linear representation helps in visualizing the arrangement of atoms.

The formula can help in predicting how the molecule might interact with others, highlighting regions that might be more reactive or participate in chemical reactions. This specific configuration not only defines the structure but also has implications for the physical and chemical properties of pentane.

Molecular Formula

The molecular formula of a compound tells us the exact number and kinds of atoms present in a single molecule. For pentane, the molecular formula is ${\text{C}}_5{\text{H}}_{12}$. This means that each molecule of pentane consists of five carbon atoms and twelve hydrogen atoms.

Unlike the structural formula, the molecular formula does not provide any information about the arrangement of these atoms. However, it is vital for understanding the composition of the molecule as a whole. Knowing the molecular formula allows chemists to calculate molar mass, which is crucial for stoichiometry in chemical reactions.

It also helps in identifying the compound and distinguishing it from other similar hydrocarbons, ensuring accurate representation in chemical equations and reactions.

Empirical Formula

The empirical formula offers the simplest whole-number ratio of the elements in a compound. For many hydrocarbons, including pentane, this formula can be identical to the molecular formula.

In the case of pentane, since the molecular formula ${\text{C}}_5{\text{H}}_{12}$ already represents the simplest ratio, the empirical formula is the same. This is because the ratio of carbon to hydrogen cannot be reduced further while still representing the compound accurately.

The empirical formula is especially useful for compounds where the molecular formula is not immediately available, or when comparing elements quantitatively in reactions, making it a key tool for chemists in analyzing and interpreting chemical data.