Met-enkephalin is one of the so-called endorphins, a class of naturally occurring morphinelike chemicals in the brain. What is the molecular mass of met-enkephalin if 20.0 mL of an aqueous solution containing 15.0 mg met-enkephalin at 298 K supports a column of water 32.9 cm high? The density of mercury at 298 K is 13.534 g/mL.

Osmotic Pressure Calculation

The osmotic pressure of a dilute solution is found to obey a relationship of the same form as the ideal gas law:

In chemistry texts, it is usually expressed in terms of the molarity of the solution and given the symbol p.

In these relationships, R = 8.3145 J/k mol is the normal gas constant and R'= 0.0821 L atm/K mol is the gas constant expressed in terms of liters and atmospheres.

For a solute mass of m = grams

of molecular weight MW = grams/mole

at absolute temperature K

dissolved to form an aqueous solution of total
volume V = cm3

the calculated osmotic pressure is

Posmotic = kPa = atmospheres.

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