8.37 We saw in Section8.8 that to pass through a channel, the ion must first lose its hydrating water molecules. To explore the motion of hydratedNa + ions, we need to know that the diffusion coefficientDof an ion is related to its mobilityuby the Einstein relation:D= uRT/zFwhere zis the charge number of the ion and Fis Faraday's constant. (a) Estimate the diffusion coefficient and the effective hydrodynamic radius a of the Na 4 ion in water at 25 ∘ C. For water,η=8.91×10 −4 kg m −1 s −1. (b) Estimate the approximate number of water molecules that are dragged along by the cations. Ionic radii are given in Table 9.3.