i recently tried to find answer for following problem:

"Assume that a membrane is permeable to Ca++ , K+ and Cl- but not to a large cation R+. The inside concentrations are [RCl] = 100mM and [CaCl2] = 200mM, and the outside concentration is [CaCl2] = 300mM. (a) Derive the Donnan equilibrium. (b) Find the steady-state equilibrium concentration for Ca++."

when i tried to write space charge neutrality equation, i encounter to a new question,
how should i consider that

a:for each Ca++ there is two Cl-  


b:each Ca++ has 2 positive charge  

in the past i only worked with the ions that had same charge or same number of molecule, for example considering we had only cl- and K+ and some large cation R+ , this situation is simple but the above mentioned problem is totally different.


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