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 composed of hydrogen and carbon. (b) The structural formula for pentane is \( \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \); both its molecular and empirical formulas are \( C_5H_{12} \).

Step-by-step solution

Define hydrocarbon

A hydrocarbon is a type of organic compound composed exclusively of hydrogen and carbon atoms. These compounds are classified further into alkanes, alkenes, alkynes, and aromatic hydrocarbons based on their structure.

Understand Pentane

Pentane is a type of alkane, which means it is a saturated hydrocarbon with single bonds only. It contains a linear or branched chain of five carbon atoms.

Draw the Structural Formula of Pentane

To draw the structural formula for pentane, arrange five carbon atoms in a straight line with each hydrogen atom bonded to the surrounding carbon atoms. \[ \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \] This is a straight-chain configuration representing n-pentane.

Determine the Molecular Formula

The molecular formula represents the actual number of atoms of each element in a molecule of a compound. For pentane, the molecular formula is \( C_5H_{12} \).

Calculate the Empirical Formula

The empirical formula represents the simplest whole number ratio of the elements in the compound. In the case of pentane, the simplest ratio is the same as the molecular formula because the subscripts cannot be further reduced. Thus, the empirical formula is \( C_5H_{12} \).

Key concepts

Organic Compounds

Organic compounds are fascinating molecules primarily composed of carbon and hydrogen atoms, often with a few other elements like oxygen, nitrogen, and sulfur. They form the basis of all life on Earth, serving as the building blocks of biomolecules such as proteins, carbohydrates, and lipids. These compounds are characterized by their carbon-based structure, which allows them to form a vast array of complex and diverse molecules through carbon's ability to form chains and rings.
Here are key characteristics of organic compounds:

• They contain carbon as a central element.
• They typically also include hydrogen, forming hydrocarbons.
• They may incorporate heteroatoms like nitrogen, oxygen, or sulfur.
• They demonstrate immense structural diversity due to carbon bonding versatility.

The classification of organic compounds includes categories such as hydrocarbons, alcohols, acids, and more, each defined by specific functional groups.
Understanding organic compounds is essential for multiple fields including chemistry, biology, and environmental science.

Alkanes

Alkanes are a crucial group of hydrocarbons known for their simplicity and stability. These organic compounds are natively saturated, meaning they only contain single carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds. Often referred to as paraffins, alkanes are generally found in fossil fuels and are important in various industrial applications.
The defining features of alkanes include:

• They are acyclic and saturated hydrocarbons.
• They have general formula \( C_nH_{2n+2} \) where \( n \) is the number of carbon atoms.
• Typically presented in straight or branched chain structures.
• They are nonpolar and generally insoluble in water, making them useful as solvents.

Because of their strong single bonds, alkanes are relatively unreactive compared to other hydrocarbons, engaging mostly in substitution reactions. Recognizing alkanes is vital for understanding the structure and reactivity of more complex hydrocarbons.

Structural Formula

A structural formula is a graphical representation of a molecule that illustrates how atoms are connected within the compound. Unlike other formula types, the structural formula gives insight into the molecule’s geometry and the spatial arrangement of atoms.
When constructing a structural formula, consider these points:

• It details the exact bonding arrangement of atoms.
• Shows the connectivity between atoms with lines representing bonds.
• May depict the geometry of molecules, including angles.
• Useful in identifying isomers, as it distinguishes different spatial arrangements.

For example, pentane's structural formula is \[ \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \] , indicating a linear sequence of five carbon atoms with sufficient hydrogen atoms ensuring each carbon forms four bonds. This simplistic illustration helps students visualize molecular structure beyond molecular formulas.

Molecular Formula

The molecular formula of a compound specifies the actual number of atoms of each element present in a single molecule. This formula provides the simplest way to convey crucial information about a substance while implying nothing about bonding or spatial arrangement, which keeps it concise.
Here are the main aspects of a molecular formula:

• It reflects the exact count of each type of atom in a molecule.
• Offers no details about the connectivity or shape of the molecule.
• Is essential for calculating molar mass and stoichiometry in reactions.

For instance, pentane is composed of five carbon atoms and twelve hydrogen atoms, thus its molecular formula is \( C_5H_{12} \). While helping in calculating the molar mass, it is often the starting point for further analysis of molecular structure and properties.