You wish to determine the molecular weight of an unknown biomolecule. It is slig

Question

You wish to determine the molecular weight of an unknown biomolecule. It is slightly soluble in hexane; you are able to dissolve 10.0 mg in 100 mL of hexane. 1) If your osmometer can measure a pressure difference as low as 0.1 Torr, what is the maximum molecular weight you could measure? 2) If your osmometer measures the difference in height across the semipermeable membrane, what is the maximum molecular weight you could measure, assuming you can detect as small a difference as 0.5 mm? The density of hexane is 0.659 g/mL at 25°C. 3) How does the solubility of your sample play a role? Would it be better to have a more soluble sample?

Explanation / Answer

Extimaton of molecular weight of compound

7. The compound dissolved in hexane

molar concentration of solution = 0.01 g/molecular weight x 0.1 L

a.

When osmometer measured a pressure of 0.1 torr

using,

pressure = iMRT

with,

i = 1, R = gas constant, T = 25 oC + 273 = 298 K

we get,

molecular weight of compound = 1 x 0.01 g x 0.0821 x 298/(0.1/760)atm x 0.1 L = 18594.01 g/ml

b.

With pressure = 0.5/760 = 0.00066 atm

so using the same equation as above,

molecular weight = 1 x 0.01 x 0.0821 x 298/(0.00066) x 0.1 = 3706.94 g/mol

c.

The solubility plays a major role in determining the colligative property. The solubility term as represented by M in the equation, is directly proportional to the pressure change seen in the system. therefore, a more soluble system would give a more reliable result.

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