Unit 3: Topic 10 - Solubility

Question 1

Is methanol, CH3OH more soluble in water, H2O, or butane, CH3CH2CH2CH3?

Answer 1

CH3OH consists of hydrogen bonds because there is a hydrogen bonded to an electronegative atom, oxygen. Since water also has a hydrogen bond (a hydrogen bonded to an oxygen), then methanol is soluble in water. Butane is held together by only London forces, and butane also has a relatively large size compared to methanol, so the intermolecular forces are relatively weak. Since methanol has strong intermolecular forces similar to water and dissimilar to butane, then methanol is more soluble in water than in butane.

Question 2

Discuss why dodecanoic acid, CH3(CH2)10COOH is less soluble in water than methanol, CH3OH, even though both compounds contain a hydrogen bond.

Answer 2

Although both compounds contain a hydrogen bond, CH3OH is mostly held by hydrogen bonds because it dominates the nonpolar CH3 methyl group. However, the nonpolar chain in dodecanoic acid is long enough to have an effect on the IMFs in general. Additionally, the size of a larger molecule makes it more difficult for a solvent to fully wrap around and dissolve it, so larger molecules are less soluble. Therefore, dodecanoic acid being less polar than methanol and combined with its large size means methanol is more soluble in water.