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Molecular mass is an essential concept in understanding chemical substances. It represents the weight of all the atoms in a molecule and is typically expressed in grams per mole (g/mol). Calculating the molecular mass requires summing the atomic masses of all atoms in a molecular formula.
For example, let's consider ethane and ethene:

• Ethane, \( ext{C}_2 ext{H}_6 \), consists of two carbon atoms and six hydrogen atoms. Each carbon atom has an atomic mass of 12.01 u, and each hydrogen atom has a mass of 1.01 u. Therefore, its molar mass is calculated as follows: \[ ext{Molar mass of ethane} = (2 \times 12.01) + (6 \times 1.01) = 30.08 \text{g/mol} \]
• Ethene, \( ext{C}_2 ext{H}_4 \), with two carbons and four hydrogens, has a molar mass of: \[ ext{Molar mass of ethene} = (2 \times 12.01) + (4 \times 1.01) = 28.06 \text{g/mol} \]

This clear difference in molecular mass explains why ethane is heavier than ethene.

The boiling point of a substance is another crucial concept in chemistry. It's the temperature at which a liquid turns into a gas. Boiling point is influenced by molecular interactions. Within a homologous series, as molecular mass increases, boiling point generally rises due to stronger van der Waals forces.
These forces are types of weak interactions that arise because of the attraction between molecules. Larger molecules have more electrons, leading to stronger van der Waals forces, and thus require more energy (higher temperature) to break the interactions, allowing the transition from liquid to gas.
However, exceptions exist due to molecular structure and shape. Branching can influence boiling point, sometimes allowing a lower boiling point despite a larger molecular mass, due to reduced surface contact and weaker intermolecular forces.

Isomers are fascinating structures in chemistry. They share the same molecular formula but differ in the arrangement of atoms, leading to different chemical properties. With butene \( ext{C}_4 ext{H}_8 \), two primary structural isomers exist based on the location of the double bond:

• 1-Butene, where the double bond is between the first and second carbon atoms.
• 2-Butene, where the double bond is between the second and third carbon atoms.

Isomers often have different physical and chemical properties despite having the same number of each type of atom. This diversity allows for a wide variety of chemical reactions and uses in different applications. Isomerism is key to the diversity of chemical compounds, especially organic molecules.

The geometry around the carbon-carbon double bond in alkenes is unique and important in understanding their chemistry. Unlike single-bonded carbons which allow for free rotation, double bonds create a planar structure that restricts this rotation.
The basic geometry for the carbon atoms involved in a double bond is known as trigonal planar. This means each carbon atom in the double bond has bond angles of about 120 degrees. It creates a flat and rigid structure, with atoms lying in the same plane.
This planar configuration is essential as it influences the physical properties and reactivity of alkenes. Knowing that the geometry is trigonal planar, not T-shaped, is crucial for predicting and understanding how alkenes react with other chemical substances.