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Topic: Physical chemistry - I don't know where I am making a mistake!  (Read 247 times)

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Offline Shaohao

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Physical chemistry - I don't know where I am making a mistake!
« on: October 06, 2020, 06:51:01 AM »
Hello everyone I have a huge request to make me aware of the mistake I make:

Namely, I have the task and substances: starch (1M), NaCl ( 0.9%), Na2SO4 ( 1M)

I am measuring with an osmometer and receive the results in turn (median values):

113,667 mOsm/kg; 272,667 mOsm/kg; 479,667 mOsm/kg

-osmometer operates -7 C ( 266 K)
- volume in ependorph of 100 ul

I calculate the theoretical values of pressure Osm with Pi = mRT (a small concentration is treated by Cn as a molar concentration):

1 starch Pi= 1 x 8.31 x 266 = 2210.46 kPa
2NaCl Pi= 2 x 8,31 x 266 = 4420,92
3 Na2SO4 Pi= 3 x 8.31 x 266 = 6631.38

Now I have to convert mOsm/kg to Pa from the values I received during the measurement ( Pi observed)

For starch:
1 mOsm - 2270 Pa ( in 273 K ) 258024,09 Pa - 273k
113,667 mOsm - x y - 266 K
x = 258024.09 Pa y = 251408.0877

for NaCl = 603083,4723 Pa and Na2SO4 =1060925,011 Pa

I calculate the isotonic factor i = Piobs/Pitorets. for NaCl and Na2SO4

NaCl- i = 0,136
Na2SO4 i=0,159

The last point is the calculation of the degree of dissociation a = ( i - 1 ) / ( n-1 ) n- l. of ions to which the substrate dissociates.

a1 = - 0,84


and there's no point in writing more, because I can't get the degree of dissociation -0.83

I think that this may be due to an erroneous conversion to Pa. I would very much like to ask for directions and possibly help in calculating the pressure

Thank you!

Offline Enthalpy

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Re: Physical chemistry - I don't know where I am making a mistake!
« Reply #1 on: October 11, 2020, 05:57:05 PM »
Starch is a polymer. What 1M means isn't clear to me, perhaps moles of monomer? And how this acts on osmosis is pretty much unclear. Does osmosis even make sense with polymers? Clog some membrane? Or the presence of the polymer has zero effect?

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