.. A dry solid containing 0.3 kg of solute/kg of inert material is subjected to multiple-stage countercurrent extraction using an aqueous solution containing 0.2% by weight of the solute as extracting agent. The solvent retained by the inert solid is constant in all stages and is equal to 1.5kg of water/kg of inert. The raffinate that leaves the system (consisting of the inert solid. The raffinate that leaves the system (consisting of the inert solid and the retained solution) contains 0.05kg of solute/5kg of inert. Calculate: a) The number of theoretical stages necessary, if the concentration of the solution that comes out as an extract is 5% by weight of solute. b) The solvent/feed ratio if the number of theoretical stages is five. c) The minimum required solvent/feed ratio.

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