Density and Boiling Points of Ethanol and Octane

[JTrudeau]

I'm going through last year's trial paper for my school, and one of the questions says "Suggest a reason why ethanol has a lower boiling point but is denser than octane", with a table listing the molar mass, boiling point, density, and heat of combustion of octane and ethanol.

I figured that octane has a higher BP because it has greater Dispersion forces, but I have no idea for the density part. Any ideas? Thanks! :3

[RuiAce]

I'm going through last year's trial paper for my school, and one of the questions says "Suggest a reason why ethanol has a lower boiling point but is denser than octane", with a table listing the molar mass, boiling point, density, and heat of combustion of octane and ethanol.

I figured that octane has a higher BP because it has greater Dispersion forces, but I have no idea for the density part. Any ideas? Thanks! :3

Density is a measure of mass of substance per volume. If something is denser, for a similar volume of content the actual mass of that substance (i.e. ethanol/octane) will be higher.

I'm not certain, but here, I reckon that strength of the intermolecular forces is once again the deciding factor and why octane has a lower density than ethanol. It's going to be easier for octane to just pack together as if zigzags were forming, whereas ethanol doesn't have that many dispersion forces. In fact, whereas the -OH group is arguably useful for dipole-dipole interactions and H-bonding, it's probably also making it harder because the structure of the molecule isn't symmetrical (and hence somewhat inconsistent), hindering the ability for molecules to pack together.

[jakesilove]

Density is a measure of mass of substance per volume. If something is denser, for a similar volume of content the actual mass of that substance (i.e. ethanol/octane) will be higher.

I'm not certain, but here, I reckon that strength of the intermolecular forces is once again the deciding factor and why octane has a lower density than ethanol. It's going to be easier for ethanol to just pack together as if zigzags were forming, whereas ethanol doesn't have that many dispersion forces. In fact, whereas the -OH group is arguably useful for dipole-dipole interactions and H-bonding, it's probably also making it harder because the structure of the molecule isn't symmetrical (and hence somewhat inconsistent), hindering the ability for molecules to pack together.

I agree with all of the above; plus, for a tricky question like that, you don't always need what's 'right'; rather, you need what makes sense

[JTrudeau]

That makes sense, thank you!