i just did like 30 of these last week and havent slept its late im brain dead

At 300 K a 500.0 mL solution containing 0.4314 g of dextrose has an osmotic pressure of 89.6 mm Hg. What is the molar mass of dextrose?

osmotic pressure = 89.6 mmHg
Molarity= x
gas constant .08206
temp kelvin 300

osmotic pressure = MRT

89.6= M (.08206) (300)

M= 3.64

I am totally forgetting how to find the molecular weight from the molarity.
Something seems off.
3.64 mole dextrose per 1 Liter solution. Since we only have .5 L, do I divide the moles as well? So, 1.82 mole dextrose = .4314 g dextrose. No way thats right..

i copied it exactly but im thinking something may be off in the problem?

This post was edited by fingerling on Feb 11 2014 01:13am

e/ sorry read wrong early morning -.-

your error is using the pressure in mmHg while the gas constant R that you are using is in atm. convert mm Hg to atm and you should be fine and get an answer around 180.16

This post was edited by cialda on Feb 11 2014 07:57am

Ah dang you're right

Thanks

What is the value of the van't Hoff factor for KCl if a 1.00m aqueous solution shows a vapor pressure depression of 0.734 mmHg at 298 ∘C? (The vapor pressure of water at 298 K is 23.76 mmHg.)


could someone help with this one as well? thanks

anyone

is the "m" molality or molarity?

also what level class is this? in an ideal solution, the van't hoff factor should just be 2 iirc.

for non ideal, i believe it should be:

(Pnot - P)/Pnot = i*mole fraction
Pnot = vapor pressure pure solvent
P = vapor pressure of the resulting solution
i = van't hoff factor
mole fraction = mole solute/total mole

This post was edited by cialda on Feb 11 2014 09:17pm

(Pnot - P)/Pnot = i*mole fraction
Pnot = 23.76
P = 23.76 - 0.734
i = unknown
mole fraction solute = mole solute/total mole

1 molality = 1 mole KCl / 1000 g H2O
1000 g H2O = 55.56 mole h2o
mole fraction = 1/(55.56+1) ~= 0.0177

((Pnot - P)/Pnot)/mole fraction = 1.747


another way to do it:

Mole fraction solute calculated (theoretical) = 1/56.56 = 0.0177

P = Pnot *mole fraction solvent
P/Pnot = 0.969 = mole fraction solvent
1 = mole fraction solvent + mole fraction solute
mole fraction solute = 1 - mole fraction solvent = 0.0309

i (vant hoff factor) = observed colligative value / calculated value with no association/dissociation
i = 0.0309 / 0.0177 = 1.747

hopefully this helps. not too familiar with these formulas myself.

This post was edited by cialda on Feb 11 2014 10:03pm