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If crystals of a relatively insoluble substance are placed in contact with a saturated solution of the substance, an exchange of ions between the solution and surface will take place. If the ions in solution are labeled with a radioactive isotope, this exchange, which depends upon the surface area of the crystals, can be determined, since at equilibrium:

\frac{ Total\, radioactive \, atoms \, on \, surfrace}{Total \, radioactive \, atoms \, in\, solution }=\frac{amount \, of \, substance\, on \, surface}{amount \, of \, substance\, in\, solution}

If the molecules are assumed to be cubic:

Area\, per\, molecule \, = \left(\frac{mol.\, wt.}{6.02\cdot10^{23}}\cdot\frac{1}{\rho}\right)^{\frac{2}{3}}cm^2

where ρ is the denisity (6.2 g/cm3 PbSO4).

Since there will be 1 radioactive atom per molecule, the specific surface area (i.e. the surface area per gram of sample) can readily be calculated.