[jmg12]

An aqueous solution contains 3.00% phenylalanine (C9H11NO2) by mass. Assume the phenylalanine is nonionic and nonvolatile. Find the osmotic pressure of the solution at 25 C.

My professor says it is 4.58 atm but I keep getting 4.44 atm. The textbook says 4.58 atm but that's impossible unless you use molality in the osmotic pressure equation which is WRONG. Please help clarify. Thank you.

If it is 3.00% by mass, doesn't that mean it is 3g while the solution is 100g therefore it should be the moles/.1L. That is what I did and I got 4.44 atm. PLEASE HELP ME.

[DevaDevil]

If it is 3.00% by mass, doesn't that mean it is 3g while the solution is 100g

3% by mass means that of the total mass of the solution (aka water plus phenylalanine) 3% is phenylalanine

[jmg12]

exactly, so shouldn't it be 97 g H2O and 3 g Phenyl? For a total of 100g solution therefore .1L is what you use when finding the molarity...

[DevaDevil]

using the correct values for density of water, molar mass, temperature, and only using significant numbers at the end, I reach a value of 4.59 atm. Close enough to 4.58 to say that answer is correct.

Molar mass = 165.19 g/mol
density of water at 25C = 0.997 g/ml
T = 298.15 K


in 100 g solution: 3g PhAla, 97g H₂O

3 g PhAla / 146.19 = 0.0181609 mol
97g H₂O = 0.096709 ml.

M = 0.0181609/0.096709 = 0.18779 M

Osmotic pressure = 1 (van t Hoff factor for non-ionic, non-volatile compound) * MRT

= 0.18779 * 8.3145 * 298.15 = 465.5 kPa = 4.59 atm

[jmg12]

Why are you using 97 g of H2O. It should be 100g of solution in the equation so .1L because 97g H2o + 3g phenyl = 100g solution which is .1L in molarity equation.

[rabolisk]

Why would a 100g solution necessarily be .1L?

[jmg12]

I thought you have to assume it because they don't give you the density for phenyl. I just don't get why the phenyl mass isn't being accounted for when you do the molarity because it is the moles divided by volume of solution.

[DevaDevil]

you do not need the density of the PhAla. Assume the PhAla is completely dissolved in the water, and it does not take up any additional volume. That way you can calculate the volume of the solution using just the mass of the water

and the PhAla mass is accounted for because you calculate the number of moles of PhAla from the mass.

SO: mass of PhAla -> moles of PhAla
Mass of water -> volume of water

moles of PhAla/volume of water = concentration of PhAla

[jmg12]

Ohhhh, so if it says "by mass", it just means it hasn't taken up any volume? If it didn't say that, would you assume the solution was 100g?

[DevaDevil]

yes, the solution is 100g, but you do not have the density of the total solution, just the density of the water.

so you take the mass of the water of the solution to calculate the volume of that water, and assume the PhAla does not take up additional volume.

[jmg12]

Oh okay. I understand now, you explained it 100x better than my condescending professor. Much appreciated.

[Borek]

Why would a 100g solution necessarily be .1L?

Sorry to jump at your post, but it gives a good starting point to what I want to add.

Why would a solution made from 97 g of water necessarily be 97 mL?

Both approaches are only approximate, and I don't see a single reason to assume one is better than another. 4.58 atm is as good/bad answer as 4.44 atm.

100g of 3% w/w sucrose solution - volume of 99.0 mL
100g of 3% w/w acetic acid solution - volume of 99.7 mL
100g of 3% w/w TRIS solution - volume of 99.5 mL
100g of 3% w/w citric acid solution - volume of 98.9 mL
100g of 3% w/w EDTA solution - volume of 98.6 mL
100g of 3% w/w aniline hydrochloride solution - volume of 99.4 mL
100g of 3% w/w diethylamine hydrochloride solution - volume of 100.1 mL
100g of 3% w/w aluminum ammonium sulfate solution - volume of 97.5 mL
100g of 3% w/w lithium hydroxide solution - volume of 96.8 mL

Solutions of inorganic salts tend to be closer to 97-98 mL, but as you see there is no reason to assume 97 mL is in any way better than 100 mL.

[rabolisk]

I agree with Borek that there is no reason to assume that the solution will be 97 mL in volume. But to be sure, I was just pointing out the "flaw" (insofar as it disagreed with the given solution) in OP's method. On a broader note, I think it's important to remember that volume isn't usually additive in solution chemistry. For example, adding 50 mL of ethanol to 50 mL of water will not yield a 100 mL solution.