Is water a solvent in this problem?

lucky_star

The problem statement, all variables and given/known data
Determine the molality of a solution of water dissolved in methanol for which the mole fraction of water is 0.25.

Hi all,
I just want to ask if water in this problem is a solvent or a solute!? Is water always a solvent even though they say "a solution of water dissolved in methanol" in the problem? Please help me clear it up.

naele

It appears you've answered your own question. Also, note the mole fraction of water in the solution.

lucky_star

Thanks!!! Yes, and that's how it get me confused. A solvent is what we have more of, and a solute is what we have less. This appears violating the common rule. I think this is an exception right? what do you think?

naele

That's basically how you look at it. Consider a typical aqueous solution, say 1M HCl. That means there's one mole of HCl in one liter of water. But water is at roughly ~55M so there's quite a bit more of it. In this case, we're told that a water/methanol solution has water at 0.25 mole fraction. By extension we know that methanol has to be 1 - 0.25 = 0.75 which of course is greater than 0.25 so we can only conclude that methanol is the solvent here.

lucky_star

Yes, that makes sense to me! Thank so much!