Osmotic pressure measurements can be used to determine the

Chapter 5, Problem 77P

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Problem 77P

Osmotic pressure measurements can be used to determine the molecular weights of largo molecules such as proteins. For a solution of large molecules to qualify as “dilute”, its molar concentration must be very low and hence the osmotic pressure can be too small to measure accurately. For this reason, the usual procedure is to measure the osmotic pressure at a variety of concentrations, then extrapolate the results to the limit of zero concentration. Here are some data* for the protein haemoglobin dissolved in water at 3°C:

Concentration (grams/liter)	Δh (cm)
5.0	2.0
16.6	6.5
32.5	12.8
43.4	17.6
54.0	22.6

The quantity Δh is the equilibrium difference in fluid level between the solution and the pure solvent, as shown in below Figure. From these measurements, determine the approximate molecular weight of hemoglobin (in grams per mole) .

Figure: An experimental arrangement for measuring osmotic pressure. Solvent flows across the membrane from left to right until the difference in fluid level, Δh, is just enough to supply theosmotic pressure.