8.37 We saw in Section 8.8 that to pass through a channel, the ion must first lose its hydrating water molecules. To explore the motion of hydrated Na + ions, we need to know that the diffusion coefficient D of an ion is related to its mobility u by the Einstein relation:D= uRT/zF​where z is the charge number of the ion and F is Faraday's constant. (a) Estimate the diffusion coefficient and the effective hydrodynamic radius aof 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.