1) why Ethanol(CH3CH2OH) soluble in water and pentanol (C5H11OH) are not soluble in water?
I know both of them have an Hydrogen bond but I don't sure about the Polarity.

2) how can I know the Surface area of Molecules In general ?
Like if I have tow Molecules :C2H5OH-CH3OH who has more big Surface area I guess is C2H5OH but I not sure why.

Thanks.

The non-polar portion of a pentane molecule is larger than that of an ethanol molecule. That essentially squeezes it out of the water.
To picture the surface area of a molecule, just think of the atoms as spheres in a line (like pearls on a bracelet or necklace). The bigger, the longer the chain, the greater the surface area.

Hi I Understand your 2) answer. But about the 1) answer pentanol has no Polarity so he not soluble in water?

CH_3 groups and CH_2 groups are hydrophobic, that is they are not soluble in water.

In ethanol, this group is quite small and hence, the hydrogen bonding 'overcomes' the hydrophobic group's insolubility in water.

While in pentanol, the hydrophobic part is larger and one hydrogen boncing alone is not enough to make it soluble in water.

Like Unknown008 says, the hydrophilicity (causes solubility in water) and the hydrophobicity (causes insolubility) are fighting each other. For short alkyl chains -- 1, 2, 3 or 4 carbons the polar OH group wins, 5 carbons or more and the carbon chain wins. Pentanol has as much polarity as does ethanol (one OH group) but there is much more non-polarity. So pentanol is still somewhat soluble in water; more so than is hexanol, and solubility drops off still further from there. If a solute is soluble to 5% we consider it soluble, less than that it's insoluble. This is arbitrary, but you have to draw the line somewhere.

Well I Understanding your answer and thanks so much to all of u.

U Explains more good then my Teacher :)

Of course the longer hydrocarbon chain is hydrophobic, but wouldn't his molecule form micelles because of the polar (OH) head? If not, why? Also, are micelles considered "soluble" in water or do their colloidal properties put them in another category?

Oil is a micelle in water. I'll let you guess the answer for your query from there :)