Solubility Ranking

In order to understand how to rank the listed molecules with respect to solubility, it must be considered that the most soluble compounds in water are polar. Water is a polar solvent, and thus poorly solutes will dissolve in it. The phrase "like dissolves like" is often used to describe this relationship between solvents and solutes.

Hexanol has a hydroxyl group (R-OH). There is hydrogen bonding that exists as a result of the O-H bond. This is the strongest type of intermolecular force. This O-H bond is described as polar as the electronegativity difference between oxygen and hydrogen is 1.24. Any value greater than 0.5 is described as a polar covalent bond. Therefore hexanol is a polar molecule.

Propane is composed strictly of carbon and hydrogen atoms. This molecule is non-polar, as the only intermolecular forces that exist are weak London dispersion forces. The electronegativity difference in a C-H bond is 0.35, which categorizes this bond as non-polar. Therefore propane is a non-polar molecule.

Methane, composed of one single carbon atom and four hydrogen atoms, is a non-polar molecule for the same reasons as propane which are listed above. However, the amount of London dispersion forces that exist between methane molecules are smaller than that of propane. This is the result of the increased chain length of propane (3 carbon atoms) when compared to the length of methane (1 carbon atom). Both propane and methane are non-polar molecules, yet propane will be more soluble in water as it is slightly "less" non-polar than methane.

The ranking for the molecules in order from most soluble in water to least soluble in water is: hexanol, propane, methane.

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