Atkins & de Paula:
Physical Chemistry for the Life Sciences
One of the most common applications of osmosis is osmometry, the measurement of molar mass of proteins and synthetic polymers from the osmotic pressure of their solutions. If the osmotic pressure (h) is known for different mass concentrations (c) then the molar mass can be calculated by plotting h/c against c and using the formula
Where M is the molar mass (kg mol-1), h is the osmotic pressure expressed as the
height of the solution (cm), c is mass concentration (g L-1),
T is temperature (K), 
is the density of the solution (g cm-3),
g is the acceleration of free fall (m s-2), and R is the gas constant (J K-1mol-1).
To calculate the molar mass enter data pairs (c, h), density and temperature, and then click the Plot tab.
To see the slope, intercept and molar mass click the Least-Squares Analysis tab. To pick off a
value on the graph, move your cursor to the desired location then click and hold.